1322191 - Implement CSS `display: flow-root` (modern clearfix)

Issues with loading CSS style sheets from the network, parsing style sheets and style attributes in HTML markup, performing the CSS cascade, selector matching, and producing correct computed values for CSS properties. Example bugs: toggling class name on an element does not restyle the element background-color property does not support hsl() values Specifications covered: CSS Cascading CSS Conditi

[Rockridge]

Fx53：CSSのdisplayプロパティでflow-rootの値をサポート。floatの解除に使える。 / 参照：https://groups.google.com/forum/#!topic/mozilla.dev.platform/bG9Kpgr5LC4