[css-forms-1] Include fieldset and legend

[lukewarlow]

Currently, fieldset default borders don't work well in dark mode in any browser, and legend is notoriously hard to style and specifically position.

It would be good to include legend in base appearance which resolves the long standing issues, and fieldset would make sense to include so its styles are in-keeping with the rest of the elements (it may have other styling issues I'm unaware of).

It's possible we can update appearance: auto fieldset to have a better border style and that's enough.

[suethepooh]

Legend should absolutely make it into base appearance. It's such a useful element but so hard to work with.

[zcorpan]

Some relevant discussion and pointers in #11981 (comment) and later comments.

Also see whatwg/html#11095