Core-65593: Remove sitemap stylesheets

[masteradhoc]

Removes the sitemap XSL stylesheet feature from core. Chrome and other browsers are deprecating and removing XSLT support (https://developer.chrome.com/docs/web-platform/deprecating-xslt), so the <?xml-stylesheet?> processing instruction pointing at /wp-sitemap.xsl / /wp-sitemap-index.xsl will stop working for site visitors and no longer serves its purpose.

After this change, generated sitemap XML no longer includes a stylesheet processing instruction, and the /wp-sitemap.xsl and /wp-sitemap-index.xsl rewrite rules and the sitemap-stylesheet query var are removed.

Backward compatibility

• WP_Sitemaps_Stylesheet is retained but deprecated. Each of its methods triggers a deprecation notice: render_stylesheet() sends a 410 Gone status and exits, and the others return an empty string.
• WP_Sitemaps_Renderer::get_sitemap_stylesheet_url() and WP_Sitemaps_Renderer::get_sitemap_index_stylesheet_url() are deprecated and return an empty string.
• The wp_sitemaps_stylesheet_url, wp_sitemaps_stylesheet_index_url, wp_sitemaps_stylesheet_content, wp_sitemaps_stylesheet_index_content and wp_sitemaps_stylesheet_css filters are deprecated and no longer have any effect. They are still applied when sitemap XML is generated, so a plugin that hooks one gets a deprecation notice rather than being silently ignored.

Trac ticket: https://core.trac.wordpress.org/ticket/65593

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementing the removal across WP_Sitemaps, WP_Sitemaps_Renderer, canonical.php, and wp-settings.php, deleting the obsolete WP_Sitemaps_Stylesheet class, and updating/removing the associated PHPUnit tests. All changes were reviewed and tested by me.

Follow-up commits by @westonruter also used Claude Code (Claude Opus 5.5) for the deprecation notices on the removed stylesheet filters and renderer URL methods, docblock fixes, and the accompanying tests. These were reviewed and directed by @westonruter.

---

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.

[github-actions]

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[github-actions]

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Props masteradhoc, peterwilsoncc, westonruter, pbiron.

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[masteradhoc]

@westonruter

This PR removes the XSLT-based sitemap stylesheet entirely. That aligns with Chrome's XSLT deprecation (https://developer.chrome.com/docs/web-platform/deprecating-xslt).

But: per Chrome's own docs, type="text/css" continues to be supported (see "XML + CSS is not being removed"). So there's an open question for core: do we want to keep some human-readable sitemap styling via a CSS-based , or is dropping styling altogether the intended outcome here?

[westonruter]

Good question. Is CSS able to style the XML sufficiently?

[masteradhoc]

@westonruter worth a try IMO. I'd though suggest we'll split it so we can first get rid of the existing implementation and then see if and what we want to style it moving forward.

[pbiron]

Good question. Is CSS able to style the XML sufficiently?

It's possible to style the XML with CSS, but it's not possible to make the "links" actually work. However, Chrome & Firefox (and other browsers, AFAIK) don't support XLink in generic XML documents (they do in SVG and MathML)...so it wouldn't be possible for a user to click on a link in the sitemap index to load a specific sitemap, nor to click on a link in a specific sitemap to get to that URL on the site.

[masteradhoc]

@westonruter are we ready to move forward for now without styling?

[pbiron]

@masteradhoc @westonruter Have you seen the alternate PR I did that removes the need for browser-support of XSLT but still renders a human-readable version of the sitemaps by doing the XSLT server-side?

#12584

[westonruter]

There is a test failure:

1) Tests_Query_Vars::testPublicQueryVarsAreAsExpected
Care should be taken when introducing new public query vars. See https://core.trac.wordpress.org/ticket/35115
Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
     47 => 'rest_route'
     48 => 'sitemap'
     49 => 'sitemap-subtype'
+    50 => 'sitemap-stylesheet'
 )

[westonruter]

All of the methods that now no-op should probably also get @since tags to document that they don't do anything anymore. It's one thing for functions to be deprecated, but often they still retain the old behavior. Here, however, the functionality is removed entirely. This should be made more clear in the docs.

[peterwilsoncc]

I've added a few notes inline but was repeating myself.

Each of the deprecated methods will need to call _deprecated_function( __METHOD__, '7.2.0' );

[masteradhoc]

@westonruter @peterwilsoncc just saw this message displayed from chrome now by default if you still use XSLT

[westonruter]

Note: If it so happens that a cached XML Sitemap is served with the <?xml-stylesheet?> PI included, but an the XSL stylesheet itself is not cached, then Firefox shows an error because it fails to parse the XSL (since it is HTML, being the homepage response):

I think this is an exceeding edge case, and it isn't much different from serving back a white screen as it would otherwise be doing now in the normal case.

[peterwilsoncc]

I've added a few notes inline, mainly the clarify that the filters don't do anything any more and possible test improvements.

[peterwilsoncc]

I've pushed my docblock suggestions and this is now ready for commit.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 64073
GitHub commit: f4350cd

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.