I gave it some thought but it's hard to say. For most of the ones I missed and mentioned in sibling threads, I can think of people who'd be aware of them:

- Probably <ruby> and related tags are used in Japanese and other spheres with such a language feature

- <base> I knew of, I thought archive.org used it but apparently not currently. Probably if you work with archived or otherwise namespaced content, you can reasonably find this a very familiar element

- <track> if you're a media person, doesn't seem that outlandish

- <dfn> I knew of basically, but just couldn't recall the names for glossary tags

- <kbd> is used all the time on Superuser (in Markdown, where HTML is valid though of course they filter most of it)

- Some others that I didn't remember, like <template> and <article>, were just stupidity. I know these...

This leaves <del> and <ins>. There are enough diffing tools, but it's not as big a category of software people constantly work on, especially since this is only for browsers. I'd not be surprised if these are the most-forgotten elements

Or does anyone still use <map>? I saw it in the HTML tutorial (linked from a Dutch magazine) that got me into programming so I know of it and remembered while doing the quiz, but it's fairly far faded from my mind and I'm not sure anyone would ever use it when, if you want this sort of functionality, you'd probably want the versatility of JavaScript

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>, <var>, <kbd> are definitely weird ones. Maybe <kbd> would be more useful with some better default styling. I think <output> has some accessibility wins but I've never seen someone use it. <dfn> is a good pick, <abbr> is also up there with questionable semantic tags. <del> and <ins> are basically tailor-made for rendering diffs. <ruby> is really useful for Japanese, doing that styling manually would be a huge pain, but niche for sure. I'm happily surprised <small> survived. I still miss <center>.

<dfn> - I was going insane trying to remember that one! I tried <def>, <definition>, <define>...

I tried the exact same three! Eventually gave up on that path, thinking "I could swear HTML had some way to have a glossary...", and of course seeing it does after revealing the answers

samp and kbd are good for marking up terminal sessions e.g. https://j4e.name/articles/marking-up-terminal-in-html/

I got <ruby> simply because I kept seeing it in the list of tags back in the days when I used W3Schools, this useless knowledge stuck with me until today. Same for <bdi> and <kbd>. No idea what they do.

If one play/work with audio/video streaming. the <track> tag will be something that came across.

And if you deal with Japanese website development, <ruby> and associated tags will came across, though less often.

<dfn> is important when writing mathematical texts. I use it all the time.

Lots of candidates, but my vote would go to <figcaption>.

Its sole purpose is to attach a caption to an image in <figure> - which basically no one uses.

I started scrolling the list at MDN, saw that there is <portal> now, then closed the tab.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/po...

I’ve used <base> a lot, but it breaks so many things. It was one of my first couple guesses

Maybe more obscure now. This tag was extremely common in early web development.