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Bronislav Klučka points out that you can't, e.g., render a calendar using the user's (locale's) first week day, currently. CSS will likely offer ways to localise rendering of dates, times, numbers, etc, but that doesn't cover everything you might need.
Can you explain further? What does "API" mean in this context? Bug 17859 suggests that content markup might be used to address the page author's intentions regarding formatting. E.g. that a page in Czech that renders a calendar in Czech could format a calendar on that page using Czech locale rules (first day of week, weekend, day names/abbreviations, etc. etc.) There may be cases where the page author intends the page the use or would like to have access to the user-agent's locale preferences. That also needs specification. The advent of the JavaScript intl extension might help in terms of "API support", but since this is an HTML bug and, as Ian points out, CSS probably needs to provide ways of doing this, a clear HTML locale model is probably implied. In my opinion, this should be based on the existing language tagging model (using the @lang attribute). I think Ian disagrees.
By "render a calendar" I mean generate <table> markup from script. By API I mean something like the JavaScript intl extensions, maybe in fact that very API.
Let's leave this particular feature for TC39 to sort through.