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.
<!DOCTYPE> informs the browser which version of HTML (or XML) you used to write the document. Doctype is a declaration, not a tag; you can also refer to it as "document type declaration", or "DTD" for short.