The memory test is missing some deprecated and non-conforming elements. The HTML spec doesn’t have a single comprehensive list either, so it can be a little tricky to define or name “all” of the elements.

For example, there are elements like nextid or isindex which don’t have element definitions but which appear in the parsing rules for legacy compatibility. These are necessary to avoid certain security issues, but the elements should not be used and in a sense don’t exist even though they are practically cemented into HTML forever.