One of the things that the words "just compromise your design" make clear is that the focus is on the designer, not the users. Which in my view is often the problem. It's especially hollow here given that much of Karellen's point is about what's best for the users.

It is for things like checkboxes. If your design requires checkboxes to be special and different from everyone else's checkboxes, your design is wrong.

Everyone else’s checkboxes aren’t all the same though.

They used to be, and it was better that way.

No, your designer sucks.

Don't reinvent well known controls. Stop doing it. You can give it a border color and an accent color (check out the new accent-color CSS property) and for the rest you leave it alone.

This is completely silly, and no one does this. If I'm going for a square design, I'll want my checkboxes to be square, not rounded squares like Firefox offers for example.

Try to extend this idea to a game menu and see how much sense it really makes to you: would anyone really want default browser checkboxes in an otherwise medieval styled game UI, for example?

Also, if you truly believe that all checkboxes in all apps should look exactly the same (per browser/OS, and except for color), then why not extend this to other aspects of design? Why even allow different sites to use different fonts? That is much harder to adjust to than a different roundness on checkbox corners.