In that case, it’s a bug in the tests:
Tag names for HTML elements may be written with any mix of lowercase and uppercase letters that are a case-insensitive match for the names of the elements given in the HTML elements section of this document; that is, tag names are case-insensitive.
W3 HTML spec - Case insensitivity in tag names and attribute names
With that said, by far the most common style for HTML tag names is
<TAG> is a distant second, and
<Tag> is almost unheard of.