In HTML, the DOCTYPE is case insensitive. The following DOCTYPEs are all valid:
<!doctype html>
<!DOCTYPE html>
<!DOCTYPE HTML>
<!DoCtYpE hTmL>
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
DOCTYPEin 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.