Page lacks the HTML doctype, thus triggering quirks mode

Specifying a doctype prevents the browser from switching to quirks mode, which can cause your page to render in unexpected ways.

How the Lighthouse doctype audit fails

Lighthouse flags pages without the <!DOCTYPE html> declaration:

Lighthouse audit showing missing doctype

How to add a doctype declaration

Add the <!DOCTYPE html> declaration to the top of your HTML document:

<!DOCTYPE html>
<html lang="en">

See MDN's Doctype page for more information.