Given a basic `.xhtml` file, such as,
```xhtml

Test

Test

```
served with `content-type: application/xhtml+xml`, Lighthouse incorrectly reports `Page lacks the HTML doctype, thus triggering quirks mode` (https://web.dev/doctype/).

> If you serve your page as XHTML (...), you do not need a DOCTYPE to enable standards mode, as such documents always use full standards mode.

https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode

Use the provided basic .xhtml document served as application/xhtml+xml to reproduce the warning, then inspect the Lighthouse doctype audit entry point that handles it. Done means XHTML documents no longer report a missing HTML doctype while documents that need one still do.

Written by the indexing model from the issue text.