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:

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.

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Last updated 2019-05-02 UTC.