18.104.22.168.7 Date and Time state
The format shown to the user is independent of the format used for form submission. Browsers are encouraged to use user interfaces that present dates and times according to the conventions of the user's preferred locale.
Should we encourage the use of the *page's* preferred locale? It would be best if we could provide a way for page authors to create a consistent user experience. Work on the ECMAScript I18N extension may eventually help here, but only for formatting displayed values---this section seems to imply that the user agent have a built-in date/time control set that may not be scriptable. Although a given user agent won't necessarily support the requested locale, we would suggest providing a <form> attribute or a field attribute to allow the page itself to request a given locale using a BCP 47 language tag.
The I18N WG feels that this could be accomplished by making section 22.214.171.124 normative.
This bug was cloned to create bug 17859 as part of operation convergence.
See also: https://www.w3.org/Bugs/Public/show_bug.cgi?id=16165
Number formats are also affected here. I think that this remains a significant gap in HTML, since having controls like dates or numbers be consistent with the page in which they appear is more pleasing to the author (and potentially less confusing to users) than having the controls be strictly in the locale of the user-agent's runtime.