anchor.diff:      _styleElement = html.StyleElement();
       html.document.head.append(_styleElement);
       final html.CssStyleSheet sheet = _styleElement.sheet;
  -
  +    final bool isWebKit = browserEngine == BrowserEngine.webkit;
  +    final bool isFirefox = browserEngine == BrowserEngine.firefox;
       // TODO(butterfly): use more efficient CSS selectors; descendant selectors
       //                  are slow. More info:
       //
  
positive.code: 
      // By default on iOS, Safari would highlight the element that's being tapped
      // on using gray background. This CSS rule disables that.
      if (browserEngine == BrowserEngine.webkit) {
        sheet.insertRule('''
  flt-glass-pane * {
    -webkit-tap-highlight-color: transparent;
  
negative.code:   left: 0;
  }''', sheet.cssRules.length);
  
      if (isWebKit) {
        sheet.insertRule(
            'flt-semantics input[type=range]::-webkit-slider-thumb {'
            '  -webkit-appearance: none;'
  