What do you mean? Most browsers support <input type="date"> natively just fine.
The `<input type=datetime-local>` on FF has a broken time-picker[1], has always been broken, and there are no plans to ever fix it, ever.
[1] In that it will let you pick a date, but not a time - the time must be manually typed into the input!
That you’re correctly using html forms won’t quickly lead to browser improvements.. so the result is that users will hate your forms. Users/your customer might possibly even think that you’re to blame, and not $browserVendor.
“This site only uses functionality provided by the HTML spec” is not a useful goal in and of itself. Using the right tool for the job, which might be JavaScript, is always more important.