Styles declared in a component's shadowRoot should not leak out of the component

[sorvell]

When using the shadowDOM shim, styles defined in component templates are leaked outside the component e.g.

< element ... >
< template >
< style >
button {
background: red;
}
...

This will style all buttons rather than buttons inside the element. When scoped stylesheets are used, this behavior is mostly mitigated. When scoped stylesheets are not supported, we can polyfill basic support by modifying rules to be scoped to the element.

[frankiefu]

This is now shimmed by toolkit when ShadowDOM polyfill is active via re-writing the selector as "element-name button."