Support stylesheets in element templates

[sorvell]

We currently process link rel="stylesheet" in an element tag but not inside an element's template. We should process both of these the same way.

[sorvell]

Currently, we're relying on HTMLImports to load stylesheet content. HTMLImports does not load content inside of templates.

Therefore to support this enhancement, we either need to change HTMLImports or we need a separate css loader.

Another alternative would be to remove support for loading styles in custom elements via elements and instead always use @import. The @import syntax is already necessary for non-cors resources.

[sorvell]

Unfortunately, if we use @import, we lose the ability to process rules in the stylesheet. We'd need a custom css loader for this.

[ebidel]

+1

[sjmiles]

In 8df92bfcb989560bae9a5b4c909ed115e0acbbe6, groundwork was performed in HTMLImports so that linked stylesheets are loaded for parsing.

[sorvell]

This has been supported for awhile now so closing this outdated issue.