KSES: Allow filter function values in inline styles.

[jigneshbhavani]

filter has been in the safecss_filter_attr() property allowlist since [52049], and
[55564] made url() values survive, but the CSS filter functions were never added to the
function-stripping expression at wp-includes/kses.php:3018. A declaration using one still
contains a ( when it reaches the check below it, so it is dropped in full and the property
is unusable in its canonical form for anyone without unfiltered_html.

Measured on trunk at [63180], PHP 8.3:

Declaration	Before	After
filter: blur(5px)	dropped	kept
filter: brightness(0.4)	dropped	kept
filter: contrast(200%)	dropped	kept
filter: drop-shadow(16px 16px 20px blue)	dropped	kept
filter: grayscale(50%)	dropped	kept
filter: hue-rotate(90deg)	kept	kept
filter: invert(75%)	dropped	kept
filter: opacity(25%)	dropped	kept
filter: saturate(30%)	dropped	kept
filter: sepia(60%)	dropped	kept
filter: blur(5px) brightness(0.4)	dropped	kept
filter: url(#svg-blur)	kept	kept
filter: none	kept	kept

hue-rotate() already passed, but not by design: [63180] added rotate for the transform
functions and \brotate( matches inside hue-rotate(90deg) because the hyphen is a word
boundary, leaving filter: hue- in the test string. It is now matched by its own name.

This is the same change [63180] made for the transform and basic shape functions. The value
is still re-emitted from the original declaration, url() values still go through
wp_kses_bad_protocol(), and the test string check is unchanged, so the security model is
the same.

Not in scope: backdrop-filter is not allowlisted at all, and the colour functions rgb(),
rgba(), hsl() and color-mix() are dropped everywhere, which is #56391.

Testing

12 of the new cases fail on trunk without the kses.php change:

1) Tests_Kses::test_safecss_filter_attr with data set #138 ('filter: blur(5px)', 'filter: blur(5px)')
...
FAILURES!
Tests: 163, Assertions: 163, Failures: 12.

With the patch the full kses group is green, 487 tests / 1562 assertions, and phpcs
passes on both changed files.

Trac ticket: https://core.trac.wordpress.org/ticket/65871

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Drafting this description and a first pass at the test cases. I measured the
before and after behaviour myself, confirmed the tests fail without the patch, ran the
kses group and phpcs, and I take responsibility for the change.

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[github-actions]

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