IE and Edge Compatibility

Most people use Chrome, Firefox, Safari, or sometimes Opera. Webkit browsers are more or less the same too, so not a big deal. So you're really just dealing with webkit and Firefox. IE and Edge are a pain in the ass. Very few people use them. Do web developers have to cater to IE and Edge users and ensure that their sites render properly in these shitty browsers? Or should you just do a user agent check and then send an alert box telling them to use a real browser instead?

IE is the only widely used browser left that still has a remotely sane UI.

Edge is a dumbed-down Chrome clone for retards.