What is the issue with the HTML Standard?

Previously the HTML specification's wording for what language to use for the presentation of dates, times, and numbers, was changed (or maybe was always?) to refer to the node's language rather than to use the browser or system locale. However, the language used for the default text of <input type=submit> and <input type=reset> buttons and (now, also, as of #10265) for the text in the default summary of a <details> (when no <summary> is present) says to use implementation defined behavior that is probably locale-specific (which I presume means matching the browser or system locale).

@annevk suggested in #10265 (comment) that this follow the node's language to match the date/time/number rules.

It does seem to me that rules for words have some substantive differences from those for dates/times/numbers so it's not obvious to me that they should be the same. But it's also worth considering what the right thing is here.