Like HTML tags, attribute names are not case sensitive, but XHTML requires lower case. Even when coding in HTML, it's a good habit to write attribute names in lower case. It looks cleaner and enhances readability.

The thing is, when text is in all caps, letters are much harder to distinguish. As a result, people have to read letter-by-letter at a noticeably slower pace. The Nielsen Norman Group states that reading speed may reduce by 10%.^[[1]] The same happens when you use a title case or capitalize letters randomly.