[Webkit-unassigned] [Bug 197769] New: "Disable cache" causes difference in resource loading behavior for CSS style vs CSS custom property

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=197769

            Bug ID: 197769
           Summary: "Disable cache" causes difference in resource loading
                    behavior for CSS style vs CSS custom property
           Product: WebKit
           Version: Safari 12
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: hello at terrymun.com

Created attachment 369549

  --> https://bugs.webkit.org/attachment.cgi?id=369549&action=review

Bug showing that when ignoring cache, CSS custom property causes resource to be re-requested

Steps to reproduce the bug:

1. Open devtools and disable cache (aka ignore cache) in Network tab
2. Set up the element so that it uses a CSS custom property to be its "background-image" property
3. Update the CSS custom property with the same value. Note that the image will be re-requested

The browser should not re-request a loaded resource when a CSS custom property is updated but whose value did not change. This behavior is not observed if the value is set using CSS property directly.

This bug cannot be reproduced if the background image is set using the "backgroundImage" property in the style object. It appears to be a bug in the implementation of CSS custom properties.

Interestingly, this bug cannot be reproduced on iOS Safari (at least from my testing on BrowserStack). This bug can also be reproduced in Google Chrome, but not on Mozilla Firefox.

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