I was at least vaguely aware of <base>. No idea what <samp>, <track>, <dfn> or <del> do. Also apparently <ruby> is not for embedding Ruby scripts: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/ru...
samp and kbd are good for marking up terminal sessions e.g. https://j4e.name/articles/marking-up-terminal-in-html/