htmldoctype

Uppercase or lowercase doctype?

When writing the HTML5 doctype what is the correct method?

<!DOCTYPE html>

or

<!doctype html>
Solution

  • In HTML, the DOCTYPE is case insensitive. The following DOCTYPEs are all valid:

    <!doctype html>
<!DOCTYPE html>
<!DOCTYPE HTML>
<!DoCtYpE hTmL>

    See w3c.org: HTML5 Syntax (doctype)

    In XML serializations (i.e. XHTML) the DOCTYPE is not required, but if you use it, DOCTYPE should be uppercase:

    <!DOCTYPE html>

    See The XML serialization of HTML5, aka ‘XHTML5’:

    Note that if you don’t uppercase DOCTYPE in an XHTML document, the XML parser will return a syntax error.

    The second part can be written in lowercase (html), uppercase (HTML) or even mixed case (hTmL) — it will still work. However, to conform to the Polyglot Markup Guidelines for HTML-Compatible XHTML Documents, it should be written in lowercase.