It's probably easier to standardize small CSS utils than it is for a date picker.

What would the API even look like?

One could want to select a single date, multiple dates, a date range, multiple date ranges.

Perhaps some dates should be disabled, but should they just be greyed out or should there be a message when the user tries to select them, and if so, how can it be provided?

Then there are questions about different calendars and locales.

Perhaps third-party date picker libraries are more appropriate here than trying to address this in standards.