DTD
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.
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General knowledge
Technical reference
- Document.doctype, a JavaScript method that returns the doctype
<!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.