>> 1
http://www.w3.org/TR/1999/REC-html401-19991224/about.html#h-1.2.1
> Element names are written in uppercase letters (e.g., BODY). Attribute names are
> written in lowercase letters (e.g., lang, onsubmit). Recall that in HTML, element
> and attribute names are case-insensitive; the convention is meant to encourage
> readability.
そのW3Cは大文字小文字の区別をしないと言っています。むしろ大文字で書いてます。
ちなみにelementを小文字にするのはXHTMLのときです。