> (looking at you date picker), not only are the defaults not great but you can't change them even if you want to. As soon as you hit one of the many brick walls when it comes to styling or changing behavior (Oh you want the week to start on Monday? Too bad)

For the start day of the week (and most suggested customizations of input[type=date]), it's a good thing that this is outside of your control. The appropriate calendar to display should be locale-dependent. The user's platform already has capabilities to configure and display this is a way most likely for the user to be able to effectively use.

The platform's date picker is better than whatever weird thing you custom built.