• Part of Block API #41236.

The follow-up work once the #42485 is merged:

• Handle HTML entities for the purposes of get_attribute and set_attribute. Define what should happen in cases such as $p->set_attribute( 'id', 'My‐id' ); and $p->get_attribute( 'id' ) when id="My‐id".
  • Add/html character reference decoder #47040
  • Tag Processor: Document and test XSS prevention in set_attribute #44447
  • Tag Processor: throw when supplied unacceptible attribute names. #44431
  • Decide whether on* attributes should be banned in set_attribute( ) to make adding inline JavaScript harder (see @peterwilsoncc's comment and the reasoning behind not banning them).
• Improve the documentation
  • A handbook page
  • New HTML Tag Processor - An in-depth tutorial developer-blog-content#75
  • Review the documentation blocks.
  • Improve description for keys in test data providers that use numeric values (example).
  • Tag processor: update @since version tags to 6.2.0 #44432
  • Expand class-level documentation for WP_HTML_Tag_Processor #44478
• New APIs
  • WIP: Introduce class for sourcing block attributes from HTML #46345
  • Tag Processor: Add bookmark invalidation logic #47559
  • WP_HTML_Processor: Add set_content_inside_balanced_tags #47036
  • Explore HTML parsing and Adoption Agency Algorithm adamziel/wordpress-develop#1
  • WP_HTML_Tag_Processor: Allow non-attribute lexical updates #47068
  • Explore adding a has_next_tag method
  • Tag Processor: Add bookmark system for tracking semantic locations in document #46018
• Basic API
  • Tag Processor: Remove the shorthand next_tag( $tag_name ) syntax #45082
  • Tag Processor: Add get_updated_html as a non-toString method of stringifying the markup #44597
  • Add a has_class method to the public WP_HTML_Tag_Processor API #46232 (rationale)
    (Try adding layout classnames to inner block wrapper #44600 (comment)))
• HTML Tag Processor: Add WP 6.3 compat layer #47933
• Find a canonical way of stringifying the processor. One that's different from (string) $w (related comment). Done: @w->get_updated_html()

Code is located in two folders:

• https://github.com/WordPress/gutenberg/tree/trunk/lib/experimental/html
• https://github.com/WordPress/gutenberg/tree/trunk/phpunit/html

cc @gziolo @dmsnell @georgeh