[css-forms-1] Should we explicitly define `::field-text` as a contenteditable?

Currently it's not defined what `::field-text` actually is, in WebKit for example I believe it's implemented as `<div contenteditable="plaintext-only">` it would be good to standardise this (or something similar).

This way behaviour can be relied upon, and the UA stylesheet can be tailored a bit more taking that into account.

Though perhaps this is more the domain for HTML spec?