[Webkit-unassigned] [Bug 245872] New: Non-matching media @imports block rendering

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=245872

            Bug ID: 245872
           Summary: Non-matching media @imports block rendering
           Product: WebKit
           Version: Safari 16
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Page Loading
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: csswizardry at gmail.com
                CC: beidson at apple.com

Steps to reproduce the problem:

1. Add `@import url('print.css') print;` to the start of a CSS file.
2. Load host HTML page in a non-print context
3. Note that print.css is assigned highest priority and blocks rendering

Problem Description:

It’s possible to specify conditions for `@import` in CSS. They’re usually media types (e.g. `print`). If we added `media=print` to a `link rel=stylesheet`, we’d see that `print.css` would be assigned Lowest priority and would not block rendering. If we apply `print` via the `@import` rule, we’d see that the file is assigned Highest priority and blocks rendering.

The spec[1] suggests that UAs might not fetch the file at all if it doesn’t match the current context, but this is not in keeping with how `media` on `link rel=styleheet` works. I would suggest parity across the two.

As always, it’s easier to explain via waterfalls. These tests show an external CSS file[2] and an inline `style` block[3] both exhibiting the same behaviour. Please ignore the wild TTFB—it’s erroneous but doesn’t influence the outcome of the tests.

1. https://w3c.github.io/csswg-drafts/css-cascade-5/#at-import
2. https://www.webpagetest.org/result/220929_AiDcDF_BZX/3/details/
3. https://www.webpagetest.org/result/220929_BiDcJH_BQ0/1/details/

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