The <base> tag can be used to alter all the links and images on the page. It also adds the ability to specify a default target for links. For example:

There's a standard HTML element for adding auto-complete to a text field: datalist.

The <q> element is an "inline version" of the more well-known <blockquote>. Interestingly, the browser will add quotes around it, so that Devs say <q>HTML is hard</q> gets rendered as Devs say “HTML is hard”.