> The built-in controls should always be used, not because they look pretty, but because they work the same across all websites.

Funny that you mention date/ time but ignored my example of the number type input, which does not work the same across all browsers.

In any case, it proves the point I was originally making; if the built in controls were extensible, we wouldn't be in the position of reinventing the entire stack because the design team wants to be unique.