HTML API: Indicate if a given tag name represents an HTML tag.

[dmsnell]

The HTML5 living specification defines three kinds of elements:

• HTML elements.
• Foreign elements, those which are part of MathML and SVG content.
• Custom elements.

The HTML API should be able to indicate if a given tag name is possibly an HTML element or not, so that dependent functions can make appropriate decisions for self-closing elements.

HTML treats the self-closing flag differently for HTML and foreign elements. In HTML elements the flag is ignored, but in foreign content the flag indicates that the tag is self-closing or void. HTML itself does not contain self-closing elements; only some void elements.

Trac ticket: Core-60390

## Questions

• How do we keep this list updated? Since HTML adds new tags…

## Description

The HTML5 living specification defines three kinds of elements:

• HTML elements.
• Foreign elements, those which are part of MathML and SVG content.
• Custom elements.

The HTML API should be able to indicate if a given tag name is possibly an HTML element or not, so that dependent functions can make appropriate decisions for self-closing elements.

HTML treats the self-closing flag differently for HTML and foreign elements. In HTML elements the flag is ignored, but in foreign content the flag indicates that the tag is self-closing or void. HTML itself does not contain self-closing elements; only some void elements.

This patch introduces WP_HTML_Processor::is_html_tag( $tag_name ) to answer this question.

[dmsnell]

This may not be necessary; based on what I'm seeing in the spec, what matters isn't whether an element is an HTML element, but whether we're inside foreign content.

​https://html.spec.whatwg.org/#parsing-main-inforeign

So maybe this shouldn't appear and instead we need to push ahead on supporting foreign content.

This should be handled by the parsing rules for foreign content, and not by the tag name.

​https://html.spec.whatwg.org/#parsing-main-inforeign