Doctype

In HTML, the doctype is the required <!doctype html> preamble found at the top of all documents. Its sole purpose is to prevent a browser from switching into so-called “quirks mode” when rendering a document; that is, the <!doctype html> doctype ensures that the browser makes a best-effort attempt at following the relevant specifications, rather than using a different rendering mode that is incompatible with some specifications.

The doctype is case-insensitive. The convention of MDN code examples is to use lowercase, but it’s also common to write it as <!DOCTYPE html>.