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:
Each Best Practices audit is weighted equally in the Lighthouse Best Practices Score. Learn more in The Best Practices score.
How to add a doctype declaration
<!DOCTYPE html> declaration to the top of your HTML document:
See MDN's Doctype page for more information.