I was trying to rewrite some UI library with html sometime ago following the W3C accessibility specs and found out a lot of patterns can’t be done with pure html and require javascript unfortunately.
If we've concluded that's it's okay to have elements that change/morph, as we seem to with the introduction of things like details, a native tab-like element feels like a glaring omission. Tabs have been a long-standing UI pattern and forcing every site to implement their own is a nightmare for accessibility. (The page you're reading is maybe already in a browser tab.)
I wouldn't be surprised if it turned out less than half of the custom tab interfaces on the web failed from an accessibility standpoint. When considering ARIA guidance, I don't even think it's possible to build an accessible version in HTML alone.
Other people have recognized it's missing. Open UI has a draft spec for it[0] and CSS Tricks has an article from 2001 about Open UI's experiments with sections for tabs[1]. I have no idea what happened on this front, though.
Accordion behavior is discussed in the article in the "Accordions / Expanding Content Panels" section:
> Use the same name attribute on all related details (like radio buttons) to restrict only one open panel at a time
And tabs can be a <details>-based accordion with some creative CSS to adjust the layout (left as an exercise for the reader, but I could write up an example if that would be helpful!)
Yes, the tabs in a tabs pattern should be keyboard navigated using arrow keys (ironically not the Tab key).
Also, the summary for the currently open details element will have the wrong state, 'expanded' instead of 'selected'. And while a set of details can now have a maximum of one open at a time, you can't ensure exactly one is always open (without JavaScript) as the tabs pattern requires.
And yes, being able to do all of these in pure HTML/CSS would be awesome. Though we are getting there with things like `details` and the newer `popover` features which should make things like rich tooltips, menu buttons, etc. a lot easier to implement. IIRC, there are also several anchor CSS properties to make positioning a lot simpler.
