[css-cascade] Provide an attribute for assigning a to a cascade layer

[mirisuzanne]

This was raised on #4969, related to the Cascade level 5 spec.

We've defined a syntax for cascade layers to be declared as code-blocks or url imports:

@layer reset url(remedy.css); 

@layer reset {
  audio[controls] { display: block; }
}

If we're allowing authors to import entire stylesheets into a layer, it seems appropriate to make that available on and '>

<link rel="stylesheet" href="remedy.css" layer="reset">

<style layer="reset">
  audio[controls] { display: block; }
style>

[frivoal]

I suspect that's not going to work, as existing user agents would just ignore the layer attribute, and assign all those styles to the default layer instead, so that's not going to be forward compatible.

For '>

">
>
@layer reset {
  audio[controls] { display: block; }
}
</style>
For  it's more useful though, as you may be trying to load into a particular layer a stylesheet that you cannot modify.
      

[xiaochengh]

@mirisuzanne These two should have the same performance (at least in Chromium). Both are captured by preload scanners.

[yoavweiss]

I think this is a problem worth solving, either by hacking a supports syntax into media (which can be useful in other contexts as well - /cc @jridgewell) or by going the