For amusement, I tried inputting <applet>, <center>, <font>, <marquee>, and <object>, but the form declared them all invalid. I vividly remember using these over a decade ago.
There are a few sites still using <frame> and <frameset>, but both are marked as invalid.
Correctly: almost all of them were removed from the HTML spec many years ago (but not <object>, that's still a valid HTML5 element).
But: invalid according to the spec doesn't mean it won't do something in a browser, browsers aren't pure spec implementations, they're applications used by humans to view content made by humans =)
There are a few sites still using <frame> and <frameset>, but both are marked as invalid.