Updates to the HTML API in 6.9

WordPress 6.9 brings an abundance of quiet improvements to the HTML API. Updates in this release mostly represent applications of the HTML API to existing code in Core; these updates increase WordPress’ reliability, improve its security hardening, and reduce maintenance burden on the project.

Major Updates WP_HTML_Processor​::​serialize_token() is now public.

The HTML Processor’s serialize_token() method returns a fully-normalized and well-formed representation of the currently-matched token. It was introduced in #62036 for WordPress 6.7 as a private method which performs the heavy-lifting for how the HTML API turns “junk” inputs into equivalent well-formed outputs. For example:

$html = ‘5 < 8