[Feature request]: Custom attribute scoped CSS

**I'm submitting a feature request**

I am not really sure if this is the right project to submit this request in, but I didn't know which one to put it in.
I am building an Aurelia adapter for Material Design and this adapter contains both custom elements and custom attributes. For each component I need to include the component's Material Design CSS file and for elements this is easy (using either `` or the viewResources decorator). But I would really like to accomplish the same thing for my custom attributes.

Start by tracing how custom elements load CSS through and the viewResources decorator, then compare that path with custom attributes. The request is complete when custom attributes can include their component's Material Design CSS in the same way as custom elements; no specific source files or tests are named.

Written by the indexing model from the issue text.