[PP-2] Support inline and async CSS

@gapple is correct, unless you're doing something tricky with the rest of the CSS files, just making some inline does nothing. Making the entire contents inline is likely to be counter-productive, you wouldn't want everthing inline all the time because then you can't make use of browser caching.

https://www.drupal.org/project/critical_css → is handling what this issue wants to do, so this is really a feature request about bringing that functionality into core.

Either way this should be postponed on #2391025: Add support for inline JS/CSS with #attached → .

Actually this is needs more info, between the contrib module and also ✨ Allow CSS to be added at end of page by rendering assets with placeholders Active , not clear what's being asked for here that's not covered by other issues.