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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.
If anyone is still wondering in 2014, please consult this:
HTML5
W3 HTML5 Spec - Doctype
A DOCTYPE must consist of the following components, in this order:
- A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".
...
Note: Despite being displayed in all caps, the spec states it is insensitive.
XHTML5
W3 HTML5 - XHTML
This specification does not define any syntax-level requirements beyond those defined for XML proper.
XML documents may contain a
DOCTYPEif desired, but this is not required to conform to this specification. This specification does not define a public or system identifier, nor provide a formal DTD.
Looking at the XML spec, it lists DOCTYPE in caps, but I can't find anything that states that 'all caps' is required (for comparison, in the HTML5 spec listed above, it is displayed in the example in all caps, but the spec explicitly states that is is case-insensitive).
Polyglot Markup
W3 Polyglot Markup - Intro
It is sometimes valuable to be able to serve HTML5 documents that are also well formed XML documents.
W3 Polyglot Markup - Doctype
Polyglot markup uses a document type declaration (DOCTYPE) specified by section 8.1.1 of [HTML5]. In addition, the DOCTYPE conforms to the following rules:
- The string DOCTYPE is in uppercase letters.
...
So, note that Polyglot Markup uses a regular HTML5 doctype, but with additions/changes. For our discussion, most notably that DOCTYPE is declared in all caps.
Summary
View the W3's HTML vs. XHTML section.
[Opinion] I wouldn't worry too much about satisfying XML compliance unless you are specifically trying to make considerations for it. For most client and JS-based server development, JSON has replaced XML.
Therefore, I can only see this really applying if you are trying to update an existing, XHTML/XML-based legacy system to co-exist with new, HTML5 functionality. If this is the case then look into the polyglot markup spec.
According to the latest spec, you should use something that is a case-insensitive match for <!DOCTYPE html>. So while browsers are required to support whatever case you prefer, it's reasonable to infer from this that <!DOCTYPE html> is the canonical case.
Either upper or lower case is "correct". However if you use web fonts and care about IE7, I'd recommend using <!DOCTYPE html> because of a bug in IE7 where web fonts sometimes fail if using <!doctype html> (e.g. in this answer).
This is why I always upper-case the doctype.
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