Mark XSLT as deprecated

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/XML/XSLT

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

Chromium, Gecko and WebKit are looking into removing XSLT support. Chromium and Gecko currently have deprecation warnings in the console when XSLT is used (via processing instruction or `XSLTProcessor()` API).

### What did you expect to see?

A note about the possible future removal from browsers.

### Do you have any supporting links, references, or citations?

https://bugzilla.mozilla.org/show_bug.cgi?id=1993933
https://issues.chromium.org/issues/435623334
https://github.com/whatwg/html/issues/11523#issuecomment-3149280766

https://developer.chrome.com/docs/web-platform/deprecating-xslt

### Do you have anything more you want to share?

There are a few more pages on MDN about XSLT.

XPath is *not* being removed.

### MDN metadata

Page report details

* Folder: `en-us/web/xml/xslt`
* MDN URL: https://developer.mozilla.org/en-US/docs/Web/XML/XSLT
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/xml/xslt/index.md
* Last commit: https://github.com/mdn/content/commit/2d19a88d0cc560f031a07585bf57f005fec02670
* Document last modified: 2025-07-07T07:44:55.000Z