Layered @import rules in <style> should be preloaded

[Xiaocheng Hu]

Created attachment 446576 [details]
layered imports not preloaded

Test case: https://purring-various-felidae.glitch.me/css-import-preload-layer.html

It tests if the following CSS imports are preloaded:

<style>
  @import url('/stylesheet?delay=1001');
  @import url('/stylesheet?delay=1002') layer;
  @import url('/stylesheet?delay=1003') layer(foo);
</style>

(Here delay=foo means the request takes foo ms to finish)

All the three should be preloaded, but as I tested in Safari Technology Preview Release 135 (Safari 15.4, WebKit 17613.1.7.3), the two layered imports are not preloaded. They start loading only after the parser-blocking script have finished, as shown by the attached screenshot.

This will hurt the loading performance when using cascade layers, since @import is currently the only way to add an external stylesheet into a layer.

[Radar WebKit Bug Importer]

<rdar://problem/86586969>

[Antti Koivisto]

Created attachment 449326 [details]
Patch

[Antti Koivisto]

Created attachment 449328 [details]
Patch

[Antti Koivisto]

Created attachment 449330 [details]
Patch

[EWS]

Committed r288099 (246113@main): <https://commits.webkit.org/246113@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 449330 [details].