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.
See also
- Definition of the DOCTYPE in the HTML specification
- Quirks Mode and Standards Mode
- Document.doctype, a JavaScript method that returns the doctype