HTML API: Add "reset insertion mode appropriately"

[jonsurrell]

​The HTML standard describes an algorithm to "reset the insertion mode appropriately" that will be necessary for the HTML API to support a number of different tags such as TABLE or SELECT.

An implementation should be added so that the HTML Processor can continue to add tag support.

​Add an implementation and tests for "reset the insertion mode appropriately".

This unblocks work to support more tags:

• #5908
• #6040

Trac ticket: https://core.trac.wordpress.org/ticket/61549

​The HTML standard describes an algorithm to "reset the insertion mode appropriately" that will be necessary for the HTML API to support a number of different tags such as TABLE or SELECT.
An implementation should be added so that the HTML Processor can continue to add tag support.

Trac ticket: Core-61549

As the HTML Processor starts to support other insertion modes outside of "IN BODY" it needs to be aware of those other modes. This patch introduces the missing insertion modes in preparation for adding that support.

Extracted as necessary prep work to the following more complete change: ​https://github.com/WordPress/wordpress-develop/pull/6020

Props @sirreal.

[dmsnell]

In 58631:

HTML API: Add missing insertion mode constants.

As the HTML Processor starts to support other insertion modes outside of
"IN BODY" it needs to be aware of those other modes. This patch
introduces the missing insertion modes in preparation for adding that
support.

Extracted as necessary prep work to the following more complete change:
​https://github.com/WordPress/wordpress-develop/pull/6020

Props jonsurrell.
See #61549.

Merged in [58631]
​https://github.com/wordpress/wordpress-develop/commit/ea3327ab6ee2cecc5ac1a006cf06927856383ab9

@sirreal I've merged in trunk after adding the new insertion modes. I hope that's helpful, as it was clouding the diff for me.

​We engaged in some discussion in Core Slack.

At this point I feel pretty good about the implementation and whatever direction you want to take it.

@dmsnell I think you wanted to move it to the HTML Processor, that's fine with me. That's where I put it originally and only moved it so it was easily testable. I'm fine with removing the internals tests and moving the implementation 👍

Noting that because of the inherent challenges in testing this change, we're going to lean on the existing html5lib test suite, and new tests introduced with additional tag support in order to verify the behavior of the implementation.

The challenge is that this is an internal implementation and it's difficult to ascertain what the tests should be verifying. The test cases are unclear, and testing it in isolation doesn't bear a direct impact on whether the HTML API is working properly or not.

Therefore in order to avoid accidentally encoding incorrect behaviors in a unit test suite on an internal details, and to avoid architecting our code around the ability to test such internal details, we're foregoing specific new tests for this code.

[dmsnell]

In 58656:

HTML API: Implement the _reset insertion mode appropriately_ algorithm.

In order to add support for the SELECT and TABLE tags in the HTML Processor, it
needs to implement the HTML algorithm named "reset the insertion mode
appropriately".

This patch implements that algorithm to unblock the additional tag support. The
algorithm resets the parsing mode after specific state changes in complicated
situations where alternative rules are in effect (such as rules governing how
the parser handles tags found within a TABLE element).

Developed in ​https://github.com/WordPress/wordpress-develop/pull/6020
Discussed in https://core.trac.wordpress.org/ticket/61549

Props dmsnell, jonsurrell.
Fixes #61549.

Merged in [58656]
​https://github.com/wordpress/wordpress-develop/commit/28930fc10907782bd560ba810b8c1f0fe4c74521