<sh>. The “Shriek” tag prompted web browsers to scream whatever was so marked at the top of its lungs. Discontinued in 2004 after too may computers were smashed with computer bats and it was found computers don’t have lungs.
<code>. This tag, formerly used to break the ENIGMA coding on messages being sent by the Germans to their Navy, was discontinued in 1998 when it was brought to the attention of the World Wide Web Consortium that World War II had ended in, like, what, 1946? 1948? Something like that and we didn’t need to check up on Germany anymore. We have Denmark peeking in on them now and then to make sure.
<kb>. The “Kibo” tag was meant to attract the attention of Usenet celebrity James “Kibo” Parry to your web page. Use of the tag has dwindled to insignificance since 2006, when Usenet was finally torn down and replaced with a Howard Johnson’s one-hour film development booth.
<dl>. Nobody has ever known what this tag is or what it’s good for. The best hypothesis is it’s related to somebody important, like <img> maybe.