Attributes and Tags
All tags and attributes must be written in lowercase (small letters). Additionally, attribute values should be lowercase when the purpose of the text therein is only to be interpreted by machines. For instances in which the data needs to be human readable, proper title capitalization should be followed.
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<a href="http://example.com/" title="Description Here">Example.com</a>
And this pair of tags,
<head> ... </HEAD>
This last example was not valid in any version of hypertext, even 30 years ago!
All of these examples would have failed XHTML Validation.
Moving on to Bitesize HTML5 – Attribute Quote Syntax