The more you use CSS, the harder this is, because you find that CSS really scales down the number of HTML tags you use. I forgot very few really easy ones — about half of the tags I missed I had never heard of before.
(In case the image doesn’t show, I named 49 HTML elements in five minutes on this quiz.)
Note: if you cut-and-paste the HTML they provide for the image above, beware — they sneak a text ad in the bottom. Mine was for “Fort Worth Dating.” Tricky.