[Webkit-unassigned] [Bug 282027] New: CSS Custom Highlight API doesn't handle certain underline styles

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=282027

            Bug ID: 282027
           Summary: CSS Custom Highlight API doesn't handle certain
                    underline styles
           Product: WebKit
           Version: Safari 18
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: samuel.h.bradshaw at gmail.com
                CC: koivisto at iki.fi

Created attachment 473029

  --> https://bugs.webkit.org/attachment.cgi?id=473029&action=review

Screenshot showing underlined text

WebKit supports the `text-underline-offset` and `text-decoration-thickness` properties in general – for example, when applied to a <span>. However, these properties don't work when styling a ::highlight() pseudo-element.

According to the working draft CSS Pseudo-Elements Module Level 4, text-decoration **and its associated properties** should be supported on ::highlight():
https://www.w3.org/TR/css-pseudo-4/#highlight-styling

This StackOverflow question has a code snippet that demonstrates the issue:
https://stackoverflow.com/q/79060854/1349044

I have tested this on Safari 18 and Safari Technology Preview 205 (WebKit 20621.1.2.111.4) on macOS, Safari 17 on iOS 17, and Safari 18 on iOS 18. The issue is present in all versions. I attempted to run the Webkit nightly build to verify the issue there as well, but wasn't able to run it, due to remote management restrictions on my work-provided Mac.

I have also tested in Chrome on macOS and on Android. Chrome does not have the issue.

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