Docs: Update WP_HTML_Processor::class_list example to runnable snippet

[sirreal]

The makes the ::class_list methods on WP_HTML_Tag_Processor and WP_HTML_Processor example snippets into runnable PHP snippets thanks to the WordPress PHP Playground.

This is achieved by using PHP interactive code fences:

```php
<?php plain code example
```

```php interactive
<?php echo "This is interactive, expected output goes here";
```
```expected-output
This is interactive, expected output goes here
```

A following expected-output code fence preloads the expected output so the demo can be viewed without needing to download and run WordPress Playground.

See WordPress/phpdoc-parser#258 and WordPress/wporg-developer#567.

This was discussed on the July 21 2026 dev chat and mentioned in a few relevant Slack channels (#core and #docs) and reception was positive.

Trac ticket: https://core.trac.wordpress.org/ticket/64896

Demo

Before

Static snapshot - including a small typo for the discerning reader!

After

Interactive!

demo-running.mov

Use of AI Tools

None

---

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

• 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]

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Props jonsurrell.

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[sirreal]

Merged in r63293.