must start with [a-z]+ and contain a hyphen and not be identical to an existing tag (such as <color-profile> from the SVG spec)
See the HTML Standard
Browsers treat them this way, but XML is case sensitive, so valid XHTML (a subset of HTML compatible with XML) must use all lower case tag names
And valid XHTML also requires all tags to be closed (EDIT: either by <tag></tag> or <tag />) even those that don’t require closing in normal HTML (e.g. <ul><li>1<li>2</ul>)
Sure, but ebook readers are not browsers, and EPUB XHTML is not HTML (despite the family resemblance).
Admittedly the differences are subtle. That’s why they provide a validator. 