[JENKINS-14749] Errors in styles.css

[basil]

This PR adds a test that fails without the change to src/main and passes with it. The test opens various common pages in HtmlUnit and verifies that there are no CSS errors (except for YUI CSS errors, which we ignore both here and in the test harness). This will prevent any new CSS that HtmlUnit cannot understand from being introduced in the future. It will also ensure that the logs stay clean going forward. To fix the existing violation, I removed the use of :has (which HtmlUnit does not understand) in favor of a simpler CSS construct.

Alternative approach that I considered

I tried setting up PostCSS Preset Env with the has-pseudo-class polyfill to allow us to continue using this modern CSS while appeasing HtmlUnit. However the polyfill has two parts: a CSS part and a JavaScript part, and HtmlUnit choked on the JavaScript part of the polyfill. So I found it simpler to eliminate the use of :has instead.

Testing done

New unit test fails without the changes to src/main and passes with them.

Proposed changelog entries

N/A

Proposed upgrade guidelines

N/A

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1. The Jira issue, if it exists, is well-described.
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3. There is automated testing or an explanation as to why this change has no tests.
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5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
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8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

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[basil]

Note that the "Alternative approach that I considered" described above in the PR description did work for transpiling media feature range notation from newer CSS to HtmlUnit-compatible older CSS, even though that approach did not work for the :has pseudo class due to the polyfill requiring both CSS and (HtmlUnit-incompatible) JavaScript. In general, I think we should be able to transpile CSS down to an HtmlUnit-compatible version in most cases using the approach in #8149.

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!