HTML API: Consistent case sensitive handling of class names

[sirreal]

Trac ticket: https://core.trac.wordpress.org/ticket/61531

The Tag Processor CSS class name methods class_list, add_class, and remove_class should be consistent regarding case sensitivity.

These methods are intended to align with CSS class names, meaning that matching should be done ASCII case-insensitive. class_list already yields lower case unique class names, but remove_class and add_class do not have similar behavior of treating case-insensitive matching classes as equal.

• add_class should only add classes that are not already present (compared ASCII case-insensitive).
• remove_class should remove all matching classes (compared ASCII case-insensitive).

This was discussed with @dmsnell on Slack here: https://wordpress.slack.com/archives/C05NFB818PQ/p1719403633636769
Related to #61520 which documents the lower-casing behavior of class_list.

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props jonsurrell.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[sirreal]

We need to be able to match the class_name case insensitively, but still add whatever class was provided.

I changed the values to be an array of one of the following shapes:

• [ self::ADD_CLASS, (string) $class_to_add ]
• [ self::REMOVE_CLASS ]
• [ self::SKIP_CLASS ]

[sirreal]

This doesn't seem like the right approach, it may not align with browser behavior.

[sirreal]

#7169 is a fresh attempt at this.