PHPUnit tests for HTML output are fragile. Attribute order changes, a trailing semicolon, and suddenly tests fail — even though the browser
  renders the same thing. WordPress 6.9 adds assertEqualHTML() to WP_UnitTestCase. It compares HTML semantically, so only real differences cause failures.

WordPress Dev's resource offers insights, tutorials, and resources for WordPress developers and website administrators. Readers can learn about WordPress development techniques, plugin development, and theme customization. With articles, tutorials, and troubleshooting guides, WordPress Dev provides  guidance and expertise for building and maintaining WordPress-powered websites.

WordPress Developer

WordPress 6.9 introduces `assertEqualHTML()` on `WP_UnitTestCase`, a semantic HTML comparison method that replaces fragile `assertSame()` string comparisons in PHPUnit tests. Unlike `assertSame()`, it normalizes attribute order, class name order, style whitespace, tag capitalization, and HTML character references — so tests only fail when markup is semantically different. The post covers practical usage for testing HTML API transformations, block render callbacks, and Interactivity API directive injection, explains the tree-based failure output format, and provides migration tips for existing test suites.

A better way to test HTML in WordPress with assertEqualHTML()

Testing Interactivity API directive injection