diff --git a/src/server/common/cssParser.ts b/src/server/common/cssParser.ts
new file mode 100644
index 0000000000..01ae864397
--- /dev/null
+++ b/src/server/common/cssParser.ts
@@ -0,0 +1,238 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as css from './cssTokenizer';
+
+// TODO: Consider giving more information, e.g. whether the argument was a quoted string or not.
+type ParsedSelectorLiteral = string;
+type ClauseCombinator = '' | '>' | '+' | '~';
+export type ParsedSelectorClause = ParsedSelectorLiteral | { css?: string, funcs: { name: string, args: ParsedSelectorList }[] };
+export type ParsedSelector = { clauses: { clause: ParsedSelectorClause, combinator: ClauseCombinator }[] };
+export type ParsedSelectorList = (ParsedSelectorLiteral | ParsedSelector)[];
+
+export function parseCSS(selector: string): ParsedSelectorList {
+  let tokens: css.CSSTokenInterface[];
+  try {
+    tokens = css.tokenize(selector);
+    if (!(tokens[tokens.length - 1] instanceof css.EOFToken))
+      tokens.push(new css.EOFToken());
+  } catch (e) {
+    const newMessage = e.message + ` while parsing selector "${selector}"`;
+    const index = (e.stack || '').indexOf(e.message);
+    if (index !== -1)
+      e.stack = e.stack.substring(0, index) + newMessage + e.stack.substring(index + e.message.length);
+    e.message = newMessage;
+    throw e;
+  }
+  const unsupportedToken = tokens.find(token => {
+    return (token instanceof css.AtKeywordToken) ||
+      (token instanceof css.BadStringToken) ||
+      (token instanceof css.BadURLToken) ||
+      (token instanceof css.ColumnToken) ||
+      (token instanceof css.CDOToken) ||
+      (token instanceof css.CDCToken) ||
+      (token instanceof css.SemicolonToken) ||
+      // TODO: Consider using these for something, e.g. to escape complex strings.
+      // For example :xpath{ (//div/bar[@attr="foo"])[2]/baz }
+      // Or this way :xpath( {complex-xpath-goes-here("hello")} )
+      (token instanceof css.OpenCurlyToken) ||
+      (token instanceof css.CloseCurlyToken) ||
+      // TODO: Consider treating these as strings?
+      (token instanceof css.URLToken) ||
+      (token instanceof css.PercentageToken);
+  });
+  if (unsupportedToken)
+    throw new Error(`Unsupported token "${unsupportedToken.toSource()}" while parsing selector "${selector}"`);
+
+  let pos = 0;
+
+  function unexpected() {
+    return new Error(`Unexpected token "${tokens[pos].toSource()}" while parsing selector "${selector}"`);
+  }
+
+  function skipWhitespace() {
+    while (tokens[pos] instanceof css.WhitespaceToken)
+      pos++;
+  }
+
+  function isIdent(p = pos) {
+    return tokens[p] instanceof css.IdentToken;
+  }
+
+  function isString(p = pos) {
+    return tokens[p] instanceof css.StringToken;
+  }
+
+  function isNumber(p = pos) {
+    return tokens[p] instanceof css.NumberToken;
+  }
+
+  function isComma(p = pos) {
+    return tokens[p] instanceof css.CommaToken;
+  }
+
+  function isCloseParen(p = pos) {
+    return tokens[p] instanceof css.CloseParenToken;
+  }
+
+  function isStar(p = pos) {
+    return (tokens[p] instanceof css.DelimToken) && tokens[p].value === '*';
+  }
+
+  function isEOF(p = pos) {
+    return tokens[p] instanceof css.EOFToken;
+  }
+
+  function isClauseCombinator(p = pos) {
+    return (tokens[p] instanceof css.DelimToken) && (['>', '+', '~'].includes(tokens[p].value));
+  }
+
+  function isSelectorClauseEnd(p = pos) {
+    return isComma(p) || isCloseParen(p) || isEOF(p) || isClauseCombinator(p) || (tokens[p] instanceof css.WhitespaceToken);
+  }
+
+  function consumeSelectorList(): ParsedSelectorList {
+    const result = [consumeSelector()];
+    while (true) {
+      skipWhitespace();
+      if (!isComma())
+        break;
+      pos++;
+      result.push(consumeSelector());
+    }
+    return result;
+  }
+
+  function consumeSelector(): ParsedSelector | ParsedSelectorLiteral {
+    skipWhitespace();
+    const result = { clauses: [{ clause: consumeSelectorClause(), combinator: '' as ClauseCombinator }] };
+    while (true) {
+      skipWhitespace();
+      if (isClauseCombinator()) {
+        result.clauses[result.clauses.length - 1].combinator = tokens[pos++].value as ClauseCombinator;
+        skipWhitespace();
+      } else if (isSelectorClauseEnd()) {
+        break;
+      }
+      result.clauses.push({ combinator: '', clause: consumeSelectorClause() });
+    }
+    if (result.clauses.length === 1 && typeof result.clauses[0].clause === 'string')
+      return result.clauses[0].clause;
+    return result;
+  }
+
+  function consumeSelectorClause(): ParsedSelectorClause {
+    // TODO: Consider symbols like `=`, `|=`, `~=`, `*=`, `/` and convert them to strings.
+    if ((isNumber() || isString() || isStar() || isIdent()) && isSelectorClauseEnd(pos + 1))
+      return isString() ? tokens[pos++].value : tokens[pos++].toSource();
+
+    let rawCSSString = '';
+    const funcs: { name: string, args: ParsedSelectorList }[] = [];
+
+    while (!isSelectorClauseEnd()) {
+      if (isIdent() || isStar()) {
+        rawCSSString += tokens[pos++].toSource();
+      } else if (tokens[pos] instanceof css.HashToken) {
+        rawCSSString += tokens[pos++].toSource();
+      } else if ((tokens[pos] instanceof css.DelimToken) && tokens[pos].value === '.') {
+        pos++;
+        if (isIdent())
+          rawCSSString += '.' + tokens[pos++].toSource();
+        else
+          throw unexpected();
+      } else if (tokens[pos] instanceof css.ColonToken) {
+        pos++;
+        if (isIdent()) {
+          if (cssFilters.has(tokens[pos].value))
+            rawCSSString += ':' + tokens[pos++].toSource();
+          else
+            funcs.push({ name: tokens[pos++].value, args: [] });
+        } else if (tokens[pos] instanceof css.FunctionToken) {
+          const name = tokens[pos++].value;
+          if (cssFunctions.has(name))
+            rawCSSString += `:${name}(${consumeCSSFunctionArgs()})`;
+          else
+            funcs.push({ name, args: consumeSelectorList() });
+          skipWhitespace();
+          if (!isCloseParen())
+            throw unexpected();
+          pos++;
+        } else {
+          throw unexpected();
+        }
+      } else if (tokens[pos] instanceof css.OpenSquareToken) {
+        rawCSSString += '[';
+        pos++;
+        while (!(tokens[pos] instanceof css.CloseSquareToken) && !isEOF())
+          rawCSSString += tokens[pos++].toSource();
+        if (!(tokens[pos] instanceof css.CloseSquareToken))
+          throw unexpected();
+        rawCSSString += ']';
+        pos++;
+      } else {
+        throw unexpected();
+      }
+    }
+    if (!rawCSSString && !funcs.length)
+      throw unexpected();
+    return { css: rawCSSString || undefined, funcs };
+  }
+
+  function consumeCSSFunctionArgs(): string {
+    let s = '';
+    while (!isCloseParen() && !isEOF())
+      s += tokens[pos++].toSource();
+    return s;
+  }
+
+  const result = consumeSelectorList();
+  if (!isEOF())
+    throw new Error(`Error while parsing selector "${selector}"`);
+  return result;
+}
+
+export function serializeSelector(selectorList: ParsedSelectorList) {
+  return selectorList.map(selector => {
+    if (typeof selector === 'string')
+      return selector;
+    return selector.clauses.map(({ clause, combinator }) => {
+      let s = '';
+      if (typeof clause === 'string') {
+        s = clause;
+      } else {
+        if (clause.css)
+          s = clause.css;
+        s = s + clause.funcs.map(func => `:${func.name}(${serializeSelector(func.args)})`).join('');
+      }
+      if (combinator)
+        s += ' ' + combinator;
+      return s;
+    }).join(' ');
+  }).join(', ');
+}
+
+const cssFilters = new Set([
+  'active', 'any-link', 'checked', 'blank', 'default', 'defined',
+  'disabled', 'empty', 'enabled', 'first', 'first-child', 'first-of-type',
+  'fullscreen', 'focus', 'focus-visible', 'focus-within', 'hover',
+  'indeterminate', 'in-range', 'invalid', 'last-child', 'last-of-type',
+  'link', 'only-child', 'only-of-type', 'optional', 'out-of-range', 'placeholder-shown',
+  'read-only', 'read-write', 'required', 'root', 'target', 'valid', 'visited',
+]);
+
+const cssFunctions = new Set([
+  'dir', 'lang', 'nth-child', 'nth-last-child', 'nth-last-of-type', 'nth-of-type',
+]);
diff --git a/src/server/common/cssTokenizer.d.ts b/src/server/common/cssTokenizer.d.ts
new file mode 100644
index 0000000000..1678e3a24b
--- /dev/null
+++ b/src/server/common/cssTokenizer.d.ts
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export class CSSTokenInterface {
+  toSource(): string;
+  value: string;
+}
+
+export function tokenize(selector: string): CSSTokenInterface[];
+
+export class IdentToken extends CSSTokenInterface {}
+export class FunctionToken extends CSSTokenInterface {}
+export class AtKeywordToken extends CSSTokenInterface {}
+export class HashToken extends CSSTokenInterface {}
+export class StringToken extends CSSTokenInterface {}
+export class BadStringToken extends CSSTokenInterface {}
+export class URLToken extends CSSTokenInterface {}
+export class BadURLToken extends CSSTokenInterface {}
+export class DelimToken extends CSSTokenInterface {}
+export class NumberToken extends CSSTokenInterface {}
+export class PercentageToken extends CSSTokenInterface {}
+export class DimensionToken extends CSSTokenInterface {}
+export class IncludeMatchToken extends CSSTokenInterface {}
+export class DashMatchToken extends CSSTokenInterface {}
+export class PrefixMatchToken extends CSSTokenInterface {}
+export class SuffixMatchToken extends CSSTokenInterface {}
+export class SubstringMatchToken extends CSSTokenInterface {}
+export class ColumnToken extends CSSTokenInterface {}
+export class WhitespaceToken extends CSSTokenInterface {}
+export class CDOToken extends CSSTokenInterface {}
+export class CDCToken extends CSSTokenInterface {}
+export class ColonToken extends CSSTokenInterface {}
+export class SemicolonToken extends CSSTokenInterface {}
+export class CommaToken extends CSSTokenInterface {}
+export class OpenParenToken extends CSSTokenInterface {}
+export class CloseParenToken extends CSSTokenInterface {}
+export class OpenSquareToken extends CSSTokenInterface {}
+export class CloseSquareToken extends CSSTokenInterface {}
+export class OpenCurlyToken extends CSSTokenInterface {}
+export class CloseCurlyToken extends CSSTokenInterface {}
+export class EOFToken extends CSSTokenInterface {}
diff --git a/src/server/common/cssTokenizer.js b/src/server/common/cssTokenizer.js
new file mode 100644
index 0000000000..e77ddf1df1
--- /dev/null
+++ b/src/server/common/cssTokenizer.js
@@ -0,0 +1,950 @@
+/*
+ * Original at https://github.com/tabatkins/parse-css
+ * licensed under http://creativecommons.org/publicdomain/zero/1.0/
+ *
+ * Modifications copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Changes from https://github.com/tabatkins/parse-css
+//   - Tabs are replaced with two spaces.
+//   - Everything not related to tokenizing - below the first exports block - is removed.
+
+// @ts-nocheck
+
+(function (root, factory) {
+    // Universal Module Definition (UMD) to support AMD, CommonJS/Node.js,
+    // Rhino, and plain browser loading.
+    if (typeof define === 'function' && define.amd) {
+        define(['exports'], factory);
+    } else if (typeof exports !== 'undefined') {
+        factory(exports);
+    } else {
+        factory(root);
+    }
+}(this, function (exports) {
+
+var between = function (num, first, last) { return num >= first && num <= last; }
+function digit(code) { return between(code, 0x30,0x39); }
+function hexdigit(code) { return digit(code) || between(code, 0x41,0x46) || between(code, 0x61,0x66); }
+function uppercaseletter(code) { return between(code, 0x41,0x5a); }
+function lowercaseletter(code) { return between(code, 0x61,0x7a); }
+function letter(code) { return uppercaseletter(code) || lowercaseletter(code); }
+function nonascii(code) { return code >= 0x80; }
+function namestartchar(code) { return letter(code) || nonascii(code) || code == 0x5f; }
+function namechar(code) { return namestartchar(code) || digit(code) || code == 0x2d; }
+function nonprintable(code) { return between(code, 0,8) || code == 0xb || between(code, 0xe,0x1f) || code == 0x7f; }
+function newline(code) { return code == 0xa; }
+function whitespace(code) { return newline(code) || code == 9 || code == 0x20; }
+function badescape(code) { return newline(code) || isNaN(code); }
+
+var maximumallowedcodepoint = 0x10ffff;
+
+var InvalidCharacterError = function(message) {
+  this.message = message;
+};
+InvalidCharacterError.prototype = new Error;
+InvalidCharacterError.prototype.name = 'InvalidCharacterError';
+
+function preprocess(str) {
+  // Turn a string into an array of code points,
+  // following the preprocessing cleanup rules.
+  var codepoints = [];
+  for(var i = 0; i < str.length; i++) {
+    var code = str.charCodeAt(i);
+    if(code == 0xd && str.charCodeAt(i+1) == 0xa) {
+      code = 0xa; i++;
+    }
+    if(code == 0xd || code == 0xc) code = 0xa;
+    if(code == 0x0) code = 0xfffd;
+    if(between(code, 0xd800, 0xdbff) && between(str.charCodeAt(i+1), 0xdc00, 0xdfff)) {
+      // Decode a surrogate pair into an astral codepoint.
+      var lead = code - 0xd800;
+      var trail = str.charCodeAt(i+1) - 0xdc00;
+      code = Math.pow(2, 16) + lead * Math.pow(2, 10) + trail;
+      i++;
+    }
+    codepoints.push(code);
+  }
+  return codepoints;
+}
+
+function stringFromCode(code) {
+  if(code <= 0xffff) return String.fromCharCode(code);
+  // Otherwise, encode astral char as surrogate pair.
+  code -= Math.pow(2, 16);
+  var lead = Math.floor(code/Math.pow(2, 10)) + 0xd800;
+  var trail = code % Math.pow(2, 10) + 0xdc00;
+  return String.fromCharCode(lead) + String.fromCharCode(trail);
+}
+
+function tokenize(str) {
+  str = preprocess(str);
+  var i = -1;
+  var tokens = [];
+  var code;
+
+  // Line number information.
+  var line = 0;
+  var column = 0;
+  // The only use of lastLineLength is in reconsume().
+  var lastLineLength = 0;
+  var incrLineno = function() {
+    line += 1;
+    lastLineLength = column;
+    column = 0;
+  };
+  var locStart = {line:line, column:column};
+
+  var codepoint = function(i) {
+    if(i >= str.length) {
+      return -1;
+    }
+    return str[i];
+  }
+  var next = function(num) {
+    if(num === undefined)
+      num = 1;
+    if(num > 3)
+      throw "Spec Error: no more than three codepoints of lookahead.";
+    return codepoint(i+num);
+  };
+  var consume = function(num) {
+    if(num === undefined)
+      num = 1;
+    i += num;
+    code = codepoint(i);
+    if(newline(code)) incrLineno();
+    else column += num;
+    //console.log('Consume '+i+' '+String.fromCharCode(code) + ' 0x' + code.toString(16));
+    return true;
+  };
+  var reconsume = function() {
+    i -= 1;
+    if (newline(code)) {
+      line -= 1;
+      column = lastLineLength;
+    } else {
+      column -= 1;
+    }
+    locStart.line = line;
+    locStart.column = column;
+    return true;
+  };
+  var eof = function(codepoint) {
+    if(codepoint === undefined) codepoint = code;
+    return codepoint == -1;
+  };
+  var donothing = function() {};
+  var parseerror = function() { console.log("Parse error at index " + i + ", processing codepoint 0x" + code.toString(16) + ".");return true; };
+
+  var consumeAToken = function() {
+    consumeComments();
+    consume();
+    if(whitespace(code)) {
+      while(whitespace(next())) consume();
+      return new WhitespaceToken;
+    }
+    else if(code == 0x22) return consumeAStringToken();
+    else if(code == 0x23) {
+      if(namechar(next()) || areAValidEscape(next(1), next(2))) {
+        var token = new HashToken();
+        if(wouldStartAnIdentifier(next(1), next(2), next(3))) token.type = "id";
+        token.value = consumeAName();
+        return token;
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x24) {
+      if(next() == 0x3d) {
+        consume();
+        return new SuffixMatchToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x27) return consumeAStringToken();
+    else if(code == 0x28) return new OpenParenToken();
+    else if(code == 0x29) return new CloseParenToken();
+    else if(code == 0x2a) {
+      if(next() == 0x3d) {
+        consume();
+        return new SubstringMatchToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x2b) {
+      if(startsWithANumber()) {
+        reconsume();
+        return consumeANumericToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x2c) return new CommaToken();
+    else if(code == 0x2d) {
+      if(startsWithANumber()) {
+        reconsume();
+        return consumeANumericToken();
+      } else if(next(1) == 0x2d && next(2) == 0x3e) {
+        consume(2);
+        return new CDCToken();
+      } else if(startsWithAnIdentifier()) {
+        reconsume();
+        return consumeAnIdentlikeToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x2e) {
+      if(startsWithANumber()) {
+        reconsume();
+        return consumeANumericToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x3a) return new ColonToken;
+    else if(code == 0x3b) return new SemicolonToken;
+    else if(code == 0x3c) {
+      if(next(1) == 0x21 && next(2) == 0x2d && next(3) == 0x2d) {
+        consume(3);
+        return new CDOToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x40) {
+      if(wouldStartAnIdentifier(next(1), next(2), next(3))) {
+        return new AtKeywordToken(consumeAName());
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x5b) return new OpenSquareToken();
+    else if(code == 0x5c) {
+      if(startsWithAValidEscape()) {
+        reconsume();
+        return consumeAnIdentlikeToken();
+      } else {
+        parseerror();
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x5d) return new CloseSquareToken();
+    else if(code == 0x5e) {
+      if(next() == 0x3d) {
+        consume();
+        return new PrefixMatchToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x7b) return new OpenCurlyToken();
+    else if(code == 0x7c) {
+      if(next() == 0x3d) {
+        consume();
+        return new DashMatchToken();
+      } else if(next() == 0x7c) {
+        consume();
+        return new ColumnToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(code == 0x7d) return new CloseCurlyToken();
+    else if(code == 0x7e) {
+      if(next() == 0x3d) {
+        consume();
+        return new IncludeMatchToken();
+      } else {
+        return new DelimToken(code);
+      }
+    }
+    else if(digit(code)) {
+      reconsume();
+      return consumeANumericToken();
+    }
+    else if(namestartchar(code)) {
+      reconsume();
+      return consumeAnIdentlikeToken();
+    }
+    else if(eof()) return new EOFToken();
+    else return new DelimToken(code);
+  };
+
+  var consumeComments = function() {
+    while(next(1) == 0x2f && next(2) == 0x2a) {
+      consume(2);
+      while(true) {
+        consume();
+        if(code == 0x2a && next() == 0x2f) {
+          consume();
+          break;
+        } else if(eof()) {
+          parseerror();
+          return;
+        }
+      }
+    }
+  };
+
+  var consumeANumericToken = function() {
+    var num = consumeANumber();
+    if(wouldStartAnIdentifier(next(1), next(2), next(3))) {
+      var token = new DimensionToken();
+      token.value = num.value;
+      token.repr = num.repr;
+      token.type = num.type;
+      token.unit = consumeAName();
+      return token;
+    } else if(next() == 0x25) {
+      consume();
+      var token = new PercentageToken();
+      token.value = num.value;
+      token.repr = num.repr;
+      return token;
+    } else {
+      var token = new NumberToken();
+      token.value = num.value;
+      token.repr = num.repr;
+      token.type = num.type;
+      return token;
+    }
+  };
+
+  var consumeAnIdentlikeToken = function() {
+    var str = consumeAName();
+    if(str.toLowerCase() == "url" && next() == 0x28) {
+      consume();
+      while(whitespace(next(1)) && whitespace(next(2))) consume();
+      if(next() == 0x22 || next() == 0x27) {
+        return new FunctionToken(str);
+      } else if(whitespace(next()) && (next(2) == 0x22 || next(2) == 0x27)) {
+        return new FunctionToken(str);
+      } else {
+        return consumeAURLToken();
+      }
+    } else if(next() == 0x28) {
+      consume();
+      return new FunctionToken(str);
+    } else {
+      return new IdentToken(str);
+    }
+  };
+
+  var consumeAStringToken = function(endingCodePoint) {
+    if(endingCodePoint === undefined) endingCodePoint = code;
+    var string = "";
+    while(consume()) {
+      if(code == endingCodePoint || eof()) {
+        return new StringToken(string);
+      } else if(newline(code)) {
+        parseerror();
+        reconsume();
+        return new BadStringToken();
+      } else if(code == 0x5c) {
+        if(eof(next())) {
+          donothing();
+        } else if(newline(next())) {
+          consume();
+        } else {
+          string += stringFromCode(consumeEscape())
+        }
+      } else {
+        string += stringFromCode(code);
+      }
+    }
+  };
+
+  var consumeAURLToken = function() {
+    var token = new URLToken("");
+    while(whitespace(next())) consume();
+    if(eof(next())) return token;
+    while(consume()) {
+      if(code == 0x29 || eof()) {
+        return token;
+      } else if(whitespace(code)) {
+        while(whitespace(next())) consume();
+        if(next() == 0x29 || eof(next())) {
+          consume();
+          return token;
+        } else {
+          consumeTheRemnantsOfABadURL();
+          return new BadURLToken();
+        }
+      } else if(code == 0x22 || code == 0x27 || code == 0x28 || nonprintable(code)) {
+        parseerror();
+        consumeTheRemnantsOfABadURL();
+        return new BadURLToken();
+      } else if(code == 0x5c) {
+        if(startsWithAValidEscape()) {
+          token.value += stringFromCode(consumeEscape());
+        } else {
+          parseerror();
+          consumeTheRemnantsOfABadURL();
+          return new BadURLToken();
+        }
+      } else {
+        token.value += stringFromCode(code);
+      }
+    }
+  };
+
+  var consumeEscape = function() {
+    // Assume the the current character is the \
+    // and the next code point is not a newline.
+    consume();
+    if(hexdigit(code)) {
+      // Consume 1-6 hex digits
+      var digits = [code];
+      for(var total = 0; total < 5; total++) {
+        if(hexdigit(next())) {
+          consume();
+          digits.push(code);
+        } else {
+          break;
+        }
+      }
+      if(whitespace(next())) consume();
+      var value = parseInt(digits.map(function(x){return String.fromCharCode(x);}).join(''), 16);
+      if( value > maximumallowedcodepoint ) value = 0xfffd;
+      return value;
+    } else if(eof()) {
+      return 0xfffd;
+    } else {
+      return code;
+    }
+  };
+
+  var areAValidEscape = function(c1, c2) {
+    if(c1 != 0x5c) return false;
+    if(newline(c2)) return false;
+    return true;
+  };
+  var startsWithAValidEscape = function() {
+    return areAValidEscape(code, next());
+  };
+
+  var wouldStartAnIdentifier = function(c1, c2, c3) {
+    if(c1 == 0x2d) {
+      return namestartchar(c2) || c2 == 0x2d || areAValidEscape(c2, c3);
+    } else if(namestartchar(c1)) {
+      return true;
+    } else if(c1 == 0x5c) {
+      return areAValidEscape(c1, c2);
+    } else {
+      return false;
+    }
+  };
+  var startsWithAnIdentifier = function() {
+    return wouldStartAnIdentifier(code, next(1), next(2));
+  };
+
+  var wouldStartANumber = function(c1, c2, c3) {
+    if(c1 == 0x2b || c1 == 0x2d) {
+      if(digit(c2)) return true;
+      if(c2 == 0x2e && digit(c3)) return true;
+      return false;
+    } else if(c1 == 0x2e) {
+      if(digit(c2)) return true;
+      return false;
+    } else if(digit(c1)) {
+      return true;
+    } else {
+      return false;
+    }
+  };
+  var startsWithANumber = function() {
+    return wouldStartANumber(code, next(1), next(2));
+  };
+
+  var consumeAName = function() {
+    var result = "";
+    while(consume()) {
+      if(namechar(code)) {
+        result += stringFromCode(code);
+      } else if(startsWithAValidEscape()) {
+        result += stringFromCode(consumeEscape());
+      } else {
+        reconsume();
+        return result;
+      }
+    }
+  };
+
+  var consumeANumber = function() {
+    var repr = [];
+    var type = "integer";
+    if(next() == 0x2b || next() == 0x2d) {
+      consume();
+      repr += stringFromCode(code);
+    }
+    while(digit(next())) {
+      consume();
+      repr += stringFromCode(code);
+    }
+    if(next(1) == 0x2e && digit(next(2))) {
+      consume();
+      repr += stringFromCode(code);
+      consume();
+      repr += stringFromCode(code);
+      type = "number";
+      while(digit(next())) {
+        consume();
+        repr += stringFromCode(code);
+      }
+    }
+    var c1 = next(1), c2 = next(2), c3 = next(3);
+    if((c1 == 0x45 || c1 == 0x65) && digit(c2)) {
+      consume();
+      repr += stringFromCode(code);
+      consume();
+      repr += stringFromCode(code);
+      type = "number";
+      while(digit(next())) {
+        consume();
+        repr += stringFromCode(code);
+      }
+    } else if((c1 == 0x45 || c1 == 0x65) && (c2 == 0x2b || c2 == 0x2d) && digit(c3)) {
+      consume();
+      repr += stringFromCode(code);
+      consume();
+      repr += stringFromCode(code);
+      consume();
+      repr += stringFromCode(code);
+      type = "number";
+      while(digit(next())) {
+        consume();
+        repr += stringFromCode(code);
+      }
+    }
+    var value = convertAStringToANumber(repr);
+    return {type:type, value:value, repr:repr};
+  };
+
+  var convertAStringToANumber = function(string) {
+    // CSS's number rules are identical to JS, afaik.
+    return +string;
+  };
+
+  var consumeTheRemnantsOfABadURL = function() {
+    while(consume()) {
+      if(code == 0x29 || eof()) {
+        return;
+      } else if(startsWithAValidEscape()) {
+        consumeEscape();
+        donothing();
+      } else {
+        donothing();
+      }
+    }
+  };
+
+
+
+  var iterationCount = 0;
+  while(!eof(next())) {
+    tokens.push(consumeAToken());
+    iterationCount++;
+    if(iterationCount > str.length*2) return "I'm infinite-looping!";
+  }
+  return tokens;
+}
+
+function CSSParserToken() { throw "Abstract Base Class"; }
+CSSParserToken.prototype.toJSON = function() {
+  return {token: this.tokenType};
+}
+CSSParserToken.prototype.toString = function() { return this.tokenType; }
+CSSParserToken.prototype.toSource = function() { return ''+this; }
+
+function BadStringToken() { return this; }
+BadStringToken.prototype = Object.create(CSSParserToken.prototype);
+BadStringToken.prototype.tokenType = "BADSTRING";
+
+function BadURLToken() { return this; }
+BadURLToken.prototype = Object.create(CSSParserToken.prototype);
+BadURLToken.prototype.tokenType = "BADURL";
+
+function WhitespaceToken() { return this; }
+WhitespaceToken.prototype = Object.create(CSSParserToken.prototype);
+WhitespaceToken.prototype.tokenType = "WHITESPACE";
+WhitespaceToken.prototype.toString = function() { return "WS"; }
+WhitespaceToken.prototype.toSource = function() { return " "; }
+
+function CDOToken() { return this; }
+CDOToken.prototype = Object.create(CSSParserToken.prototype);
+CDOToken.prototype.tokenType = "CDO";
+CDOToken.prototype.toSource = function() { return "<!--"; }
+
+function CDCToken() { return this; }
+CDCToken.prototype = Object.create(CSSParserToken.prototype);
+CDCToken.prototype.tokenType = "CDC";
+CDCToken.prototype.toSource = function() { return "-->"; }
+
+function ColonToken() { return this; }
+ColonToken.prototype = Object.create(CSSParserToken.prototype);
+ColonToken.prototype.tokenType = ":";
+
+function SemicolonToken() { return this; }
+SemicolonToken.prototype = Object.create(CSSParserToken.prototype);
+SemicolonToken.prototype.tokenType = ";";
+
+function CommaToken() { return this; }
+CommaToken.prototype = Object.create(CSSParserToken.prototype);
+CommaToken.prototype.tokenType = ",";
+
+function GroupingToken() { throw "Abstract Base Class"; }
+GroupingToken.prototype = Object.create(CSSParserToken.prototype);
+
+function OpenCurlyToken() { this.value = "{"; this.mirror = "}"; return this; }
+OpenCurlyToken.prototype = Object.create(GroupingToken.prototype);
+OpenCurlyToken.prototype.tokenType = "{";
+
+function CloseCurlyToken() { this.value = "}"; this.mirror = "{"; return this; }
+CloseCurlyToken.prototype = Object.create(GroupingToken.prototype);
+CloseCurlyToken.prototype.tokenType = "}";
+
+function OpenSquareToken() { this.value = "["; this.mirror = "]"; return this; }
+OpenSquareToken.prototype = Object.create(GroupingToken.prototype);
+OpenSquareToken.prototype.tokenType = "[";
+
+function CloseSquareToken() { this.value = "]"; this.mirror = "["; return this; }
+CloseSquareToken.prototype = Object.create(GroupingToken.prototype);
+CloseSquareToken.prototype.tokenType = "]";
+
+function OpenParenToken() { this.value = "("; this.mirror = ")"; return this; }
+OpenParenToken.prototype = Object.create(GroupingToken.prototype);
+OpenParenToken.prototype.tokenType = "(";
+
+function CloseParenToken() { this.value = ")"; this.mirror = "("; return this; }
+CloseParenToken.prototype = Object.create(GroupingToken.prototype);
+CloseParenToken.prototype.tokenType = ")";
+
+function IncludeMatchToken() { return this; }
+IncludeMatchToken.prototype = Object.create(CSSParserToken.prototype);
+IncludeMatchToken.prototype.tokenType = "~=";
+
+function DashMatchToken() { return this; }
+DashMatchToken.prototype = Object.create(CSSParserToken.prototype);
+DashMatchToken.prototype.tokenType = "|=";
+
+function PrefixMatchToken() { return this; }
+PrefixMatchToken.prototype = Object.create(CSSParserToken.prototype);
+PrefixMatchToken.prototype.tokenType = "^=";
+
+function SuffixMatchToken() { return this; }
+SuffixMatchToken.prototype = Object.create(CSSParserToken.prototype);
+SuffixMatchToken.prototype.tokenType = "$=";
+
+function SubstringMatchToken() { return this; }
+SubstringMatchToken.prototype = Object.create(CSSParserToken.prototype);
+SubstringMatchToken.prototype.tokenType = "*=";
+
+function ColumnToken() { return this; }
+ColumnToken.prototype = Object.create(CSSParserToken.prototype);
+ColumnToken.prototype.tokenType = "||";
+
+function EOFToken() { return this; }
+EOFToken.prototype = Object.create(CSSParserToken.prototype);
+EOFToken.prototype.tokenType = "EOF";
+EOFToken.prototype.toSource = function() { return ""; }
+
+function DelimToken(code) {
+  this.value = stringFromCode(code);
+  return this;
+}
+DelimToken.prototype = Object.create(CSSParserToken.prototype);
+DelimToken.prototype.tokenType = "DELIM";
+DelimToken.prototype.toString = function() { return "DELIM("+this.value+")"; }
+DelimToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  return json;
+}
+DelimToken.prototype.toSource = function() {
+  if(this.value == "\\")
+    return "\\\n";
+  else
+    return this.value;
+}
+
+function StringValuedToken() { throw "Abstract Base Class"; }
+StringValuedToken.prototype = Object.create(CSSParserToken.prototype);
+StringValuedToken.prototype.ASCIIMatch = function(str) {
+  return this.value.toLowerCase() == str.toLowerCase();
+}
+StringValuedToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  return json;
+}
+
+function IdentToken(val) {
+  this.value = val;
+}
+IdentToken.prototype = Object.create(StringValuedToken.prototype);
+IdentToken.prototype.tokenType = "IDENT";
+IdentToken.prototype.toString = function() { return "IDENT("+this.value+")"; }
+IdentToken.prototype.toSource = function() {
+  return escapeIdent(this.value);
+}
+
+function FunctionToken(val) {
+  this.value = val;
+  this.mirror = ")";
+}
+FunctionToken.prototype = Object.create(StringValuedToken.prototype);
+FunctionToken.prototype.tokenType = "FUNCTION";
+FunctionToken.prototype.toString = function() { return "FUNCTION("+this.value+")"; }
+FunctionToken.prototype.toSource = function() {
+  return escapeIdent(this.value) + "(";
+}
+
+function AtKeywordToken(val) {
+  this.value = val;
+}
+AtKeywordToken.prototype = Object.create(StringValuedToken.prototype);
+AtKeywordToken.prototype.tokenType = "AT-KEYWORD";
+AtKeywordToken.prototype.toString = function() { return "AT("+this.value+")"; }
+AtKeywordToken.prototype.toSource = function() {
+  return "@" + escapeIdent(this.value);
+}
+
+function HashToken(val) {
+  this.value = val;
+  this.type = "unrestricted";
+}
+HashToken.prototype = Object.create(StringValuedToken.prototype);
+HashToken.prototype.tokenType = "HASH";
+HashToken.prototype.toString = function() { return "HASH("+this.value+")"; }
+HashToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  json.type = this.type;
+  return json;
+}
+HashToken.prototype.toSource = function() {
+  if(this.type == "id") {
+    return "#" + escapeIdent(this.value);
+  } else {
+    return "#" + escapeHash(this.value);
+  }
+}
+
+function StringToken(val) {
+  this.value = val;
+}
+StringToken.prototype = Object.create(StringValuedToken.prototype);
+StringToken.prototype.tokenType = "STRING";
+StringToken.prototype.toString = function() {
+  return '"' + escapeString(this.value) + '"';
+}
+
+function URLToken(val) {
+  this.value = val;
+}
+URLToken.prototype = Object.create(StringValuedToken.prototype);
+URLToken.prototype.tokenType = "URL";
+URLToken.prototype.toString = function() { return "URL("+this.value+")"; }
+URLToken.prototype.toSource = function() {
+  return 'url("' + escapeString(this.value) + '")';
+}
+
+function NumberToken() {
+  this.value = null;
+  this.type = "integer";
+  this.repr = "";
+}
+NumberToken.prototype = Object.create(CSSParserToken.prototype);
+NumberToken.prototype.tokenType = "NUMBER";
+NumberToken.prototype.toString = function() {
+  if(this.type == "integer")
+    return "INT("+this.value+")";
+  return "NUMBER("+this.value+")";
+}
+NumberToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  json.type = this.type;
+  json.repr = this.repr;
+  return json;
+}
+NumberToken.prototype.toSource = function() { return this.repr; };
+
+function PercentageToken() {
+  this.value = null;
+  this.repr = "";
+}
+PercentageToken.prototype = Object.create(CSSParserToken.prototype);
+PercentageToken.prototype.tokenType = "PERCENTAGE";
+PercentageToken.prototype.toString = function() { return "PERCENTAGE("+this.value+")"; }
+PercentageToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  json.repr = this.repr;
+  return json;
+}
+PercentageToken.prototype.toSource = function() { return this.repr + "%"; }
+
+function DimensionToken() {
+  this.value = null;
+  this.type = "integer";
+  this.repr = "";
+  this.unit = "";
+}
+DimensionToken.prototype = Object.create(CSSParserToken.prototype);
+DimensionToken.prototype.tokenType = "DIMENSION";
+DimensionToken.prototype.toString = function() { return "DIM("+this.value+","+this.unit+")"; }
+DimensionToken.prototype.toJSON = function() {
+  var json = this.constructor.prototype.constructor.prototype.toJSON.call(this);
+  json.value = this.value;
+  json.type = this.type;
+  json.repr = this.repr;
+  json.unit = this.unit;
+  return json;
+}
+DimensionToken.prototype.toSource = function() {
+  var source = this.repr;
+  var unit = escapeIdent(this.unit);
+  if(unit[0].toLowerCase() == "e" && (unit[1] == "-" || between(unit.charCodeAt(1), 0x30, 0x39))) {
+    // Unit is ambiguous with scinot
+    // Remove the leading "e", replace with escape.
+    unit = "\\65 " + unit.slice(1, unit.length);
+  }
+  return source+unit;
+}
+
+function escapeIdent(string) {
+  string = ''+string;
+  var result = '';
+  var firstcode = string.charCodeAt(0);
+  for(var i = 0; i < string.length; i++) {
+    var code = string.charCodeAt(i);
+    if(code == 0x0) {
+      throw new InvalidCharacterError('Invalid character: the input contains U+0000.');
+    }
+
+    if(
+      between(code, 0x1, 0x1f) || code == 0x7f ||
+      (i == 0 && between(code, 0x30, 0x39)) ||
+      (i == 1 && between(code, 0x30, 0x39) && firstcode == 0x2d)
+    ) {
+      result += '\\' + code.toString(16) + ' ';
+    } else if(
+      code >= 0x80 ||
+      code == 0x2d ||
+      code == 0x5f ||
+      between(code, 0x30, 0x39) ||
+      between(code, 0x41, 0x5a) ||
+      between(code, 0x61, 0x7a)
+    ) {
+      result += string[i];
+    } else {
+      result += '\\' + string[i];
+    }
+  }
+  return result;
+}
+
+function escapeHash(string) {
+  // Escapes the contents of "unrestricted"-type hash tokens.
+  // Won't preserve the ID-ness of "id"-type hash tokens;
+  // use escapeIdent() for that.
+  string = ''+string;
+  var result = '';
+  var firstcode = string.charCodeAt(0);
+  for(var i = 0; i < string.length; i++) {
+    var code = string.charCodeAt(i);
+    if(code == 0x0) {
+      throw new InvalidCharacterError('Invalid character: the input contains U+0000.');
+    }
+
+    if(
+      code >= 0x80 ||
+      code == 0x2d ||
+      code == 0x5f ||
+      between(code, 0x30, 0x39) ||
+      between(code, 0x41, 0x5a) ||
+      between(code, 0x61, 0x7a)
+    ) {
+      result += string[i];
+    } else {
+      result += '\\' + code.toString(16) + ' ';
+    }
+  }
+  return result;
+}
+
+function escapeString(string) {
+  string = ''+string;
+  var result = '';
+  for(var i = 0; i < string.length; i++) {
+    var code = string.charCodeAt(i);
+
+    if(code == 0x0) {
+      throw new InvalidCharacterError('Invalid character: the input contains U+0000.');
+    }
+
+    if(between(code, 0x1, 0x1f) || code == 0x7f) {
+      result += '\\' + code.toString(16) + ' ';
+    } else if(code == 0x22 || code == 0x5c) {
+      result += '\\' + string[i];
+    } else {
+      result += string[i];
+    }
+  }
+  return result;
+}
+
+// Exportation.
+exports.tokenize = tokenize;
+exports.IdentToken = IdentToken;
+exports.FunctionToken = FunctionToken;
+exports.AtKeywordToken = AtKeywordToken;
+exports.HashToken = HashToken;
+exports.StringToken = StringToken;
+exports.BadStringToken = BadStringToken;
+exports.URLToken = URLToken;
+exports.BadURLToken = BadURLToken;
+exports.DelimToken = DelimToken;
+exports.NumberToken = NumberToken;
+exports.PercentageToken = PercentageToken;
+exports.DimensionToken = DimensionToken;
+exports.IncludeMatchToken = IncludeMatchToken;
+exports.DashMatchToken = DashMatchToken;
+exports.PrefixMatchToken = PrefixMatchToken;
+exports.SuffixMatchToken = SuffixMatchToken;
+exports.SubstringMatchToken = SubstringMatchToken;
+exports.ColumnToken = ColumnToken;
+exports.WhitespaceToken = WhitespaceToken;
+exports.CDOToken = CDOToken;
+exports.CDCToken = CDCToken;
+exports.ColonToken = ColonToken;
+exports.SemicolonToken = SemicolonToken;
+exports.CommaToken = CommaToken;
+exports.OpenParenToken = OpenParenToken;
+exports.CloseParenToken = CloseParenToken;
+exports.OpenSquareToken = OpenSquareToken;
+exports.CloseSquareToken = CloseSquareToken;
+exports.OpenCurlyToken = OpenCurlyToken;
+exports.CloseCurlyToken = CloseCurlyToken;
+exports.EOFToken = EOFToken;
+exports.CSSParserToken = CSSParserToken;
+exports.GroupingToken = GroupingToken;
+
+}));
diff --git a/test/css-parser.spec.ts b/test/css-parser.spec.ts
new file mode 100644
index 0000000000..ab4b129c1b
--- /dev/null
+++ b/test/css-parser.spec.ts
@@ -0,0 +1,99 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { it, expect } from './fixtures';
+import * as path from 'path';
+
+const { parseCSS, serializeSelector: serialize } =
+    require(path.join(__dirname, '..', 'lib', 'server', 'common', 'cssParser'));
+
+it('should parse css', async () => {
+  expect(serialize(parseCSS('div'))).toBe('div');
+  expect(serialize(parseCSS('div.class'))).toBe('div.class');
+  expect(serialize(parseCSS('.class'))).toBe('.class');
+  expect(serialize(parseCSS('#id'))).toBe('#id');
+  expect(serialize(parseCSS('.class#id'))).toBe('.class#id');
+  expect(serialize(parseCSS('div#id.class'))).toBe('div#id.class');
+  expect(serialize(parseCSS('*'))).toBe('*');
+  expect(serialize(parseCSS('*div'))).toBe('*div');
+  expect(serialize(parseCSS('div[attr *= foo i]'))).toBe('div[attr *= foo i]');
+  expect(serialize(parseCSS('div[attr~="Bar baz"  ]'))).toBe('div[attr~="Bar baz" ]');
+  expect(serialize(parseCSS(`div    [ foo = 'bar'  s]`))).toBe(`div [ foo = "bar" s]`);
+
+  expect(serialize(parseCSS(':hover'))).toBe(':hover');
+  expect(serialize(parseCSS('div:hover'))).toBe('div:hover');
+  expect(serialize(parseCSS('#id:active:hover'))).toBe('#id:active:hover');
+  expect(serialize(parseCSS(':dir(ltr)'))).toBe(':dir(ltr)');
+  expect(serialize(parseCSS('#foo-bar.cls:nth-child(3n + 10)'))).toBe('#foo-bar.cls:nth-child(3n + 10)');
+  expect(serialize(parseCSS(':lang(en)'))).toBe(':lang(en)');
+  expect(serialize(parseCSS('*:hover'))).toBe('*:hover');
+
+  expect(serialize(parseCSS('div span'))).toBe('div span');
+  expect(serialize(parseCSS('div>span'))).toBe('div > span');
+  expect(serialize(parseCSS('div +span'))).toBe('div + span');
+  expect(serialize(parseCSS('div~ span'))).toBe('div ~ span');
+  expect(serialize(parseCSS('div   >.class #id+ span'))).toBe('div > .class #id + span');
+  expect(serialize(parseCSS('div>span+.class'))).toBe('div > span + .class');
+
+  expect(serialize(parseCSS('div:not(span)'))).toBe('div:not(span)');
+  expect(serialize(parseCSS(':not(span)#id'))).toBe('#id:not(span)');
+  expect(serialize(parseCSS('div:not(span):hover'))).toBe('div:hover:not(span)');
+  expect(serialize(parseCSS('div:has(span):hover'))).toBe('div:hover:has(span)');
+  expect(serialize(parseCSS('div:right-of(span):hover'))).toBe('div:hover:right-of(span)');
+  expect(serialize(parseCSS(':right-of(span):react(foobar)'))).toBe(':right-of(span):react(foobar)');
+  expect(serialize(parseCSS('div:is(span):hover'))).toBe('div:hover:is(span)');
+  expect(serialize(parseCSS('div:scope:hover'))).toBe('div:hover:scope()');
+
+  expect(serialize(parseCSS(':text("foo")'))).toBe(':text(foo)');
+  expect(serialize(parseCSS(':text("*")'))).toBe(':text(*)');
+  expect(serialize(parseCSS(':text(*)'))).toBe(':text(*)');
+  expect(serialize(parseCSS(':text("foo", normalize-space)'))).toBe(':text(foo, normalize-space)');
+  expect(serialize(parseCSS(':index(3, div    span)'))).toBe(':index(3, div span)');
+  expect(serialize(parseCSS(':is(foo, bar>baz.cls+:not(qux))'))).toBe(':is(foo, bar > baz.cls + :not(qux))');
+  // expect(serialize(parseCSS(':right-of(div, bar=50)'))).toBe(':right-of(div, bar=50)');
+});
+
+it('should throw on malformed css', async () => {
+  function expectError(selector: string) {
+    let error = { message: '' };
+    try {
+      parseCSS(selector);
+    } catch (e) {
+      error = e;
+    }
+    expect(error.message).toContain(`while parsing selector "${selector}"`);
+  }
+
+  expectError('');
+  expectError('.');
+  expectError('#');
+  expectError('..');
+  expectError('#.');
+  expectError('.#');
+  expectError('[attr=');
+  expectError(':not(div');
+  expectError('div)');
+  expectError('()');
+  expectError(':not(##)');
+  expectError(':not()');
+  expectError(':not(.)');
+  expectError('div,');
+  expectError(',div');
+  expectError('div,,span');
+  expectError('div > > span');
+  expectError('div > > > > span');
+  expectError('div >');
+});
diff --git a/test/selectors-css.spec.ts b/test/selectors-css.spec.ts
index a2fc9faa4f..9f83efa248 100644
--- a/test/selectors-css.spec.ts
+++ b/test/selectors-css.spec.ts
@@ -131,3 +131,39 @@ it('should work with numerical id', async ({page, server}) => {
   const element = await page.$('#\\31\\32\\33');
   expect(element).toBeTruthy();
 });
+
+it('should work with :nth-child', async ({page, server}) => {
+  await page.goto(server.PREFIX + '/deep-shadow.html');
+  expect(await page.$$eval(`css=span:nth-child(odd)`, els => els.length)).toBe(3);
+  expect(await page.$$eval(`css=span:nth-child(even)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:nth-child(n+1)`, els => els.length)).toBe(4);
+  expect(await page.$$eval(`css=span:nth-child(n+2)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:nth-child(2n)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:nth-child(2n+1)`, els => els.length)).toBe(3);
+  expect(await page.$$eval(`css=span:nth-child(-n)`, els => els.length)).toBe(0);
+  expect(await page.$$eval(`css=span:nth-child(-n+1)`, els => els.length)).toBe(3);
+  expect(await page.$$eval(`css=span:nth-child(-n+2)`, els => els.length)).toBe(4);
+  expect(await page.$$eval(`css=span:nth-child(23n+2)`, els => els.length)).toBe(1);
+});
+
+it('should work with :not', async ({page, server}) => {
+  await page.goto(server.PREFIX + '/deep-shadow.html');
+  expect(await page.$$eval(`css=div:not(#root1)`, els => els.length)).toBe(2);
+  expect(await page.$$eval(`css=body :not(span)`, els => els.length)).toBe(4);
+  expect(await page.$$eval(`css=div > :not(span):not(div)`, els => els.length)).toBe(0);
+});
+
+it('should work with spaces in :nth-child and :not', test => {
+  test.fixme('Our selector parser is broken');
+}, async ({page, server}) => {
+  await page.goto(server.PREFIX + '/deep-shadow.html');
+  expect(await page.$$eval(`css=span:nth-child(23n +2)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:nth-child(23n+ 2)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:nth-child( 23n + 2 )`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:not(#root1 #target)`, els => els.length)).toBe(3);
+  expect(await page.$$eval(`css=span:not(:not(#root1 #target))`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span:not(span:not(#root1 #target))`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=div > :not(span)`, els => els.length)).toBe(2);
+  expect(await page.$$eval(`css=body :not(span, div)`, els => els.length)).toBe(1);
+  expect(await page.$$eval(`css=span, section:not(span, div)`, els => els.length)).toBe(5);
+});
diff --git a/tsconfig.json b/tsconfig.json
index 8b6528ed5a..4606fdec4c 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -7,9 +7,10 @@
     "rootDir": "./src",
     "outDir": "./lib",
     "strict": true,
+    "allowJs": true,
     "declaration": false
   },
   "compileOnSave": true,
-  "include": ["src/**/*.ts"],
-  "exclude": ["node_modules"]
+  "include": ["src/**/*.ts", "src/**/*.js"],
+  "exclude": ["node_modules", "src/.eslintrc.js"]
 }