Render-blocking after link[rel=stylesheet]

As was recently specced, in-body stylesheets block rendering for subsequent elements:

```html

I render immediately

I wait for the above CSS

My experiments with the polyfill suggest it doesn’t replicate this behavior, and I couldn’t find anything in the spec about it (it does mention parser blocking, but not render blocking).

The behavior would probably have to be slightly different, only blocking render for subsequent elements inside the streamed-in element container. Do you think this would be worth supporting? I’d love to see these two streaming performance techniques coexist.

Start by reviewing the streaming-include polyfill and comparing its behavior with the issue's in-body stylesheet example and the referenced parser-blocking specification. Determine whether subsequent elements in the streamed-in container can be render-blocked, and document or test the behavior before deciding whether support is feasible.

Written by the indexing model from the issue text.