Why some big sites like Facebook or eBay don’t use client side validation for login? [on hold]

Most UI experts recommend using client side validation for a better user experience.

But why do some of the big sites like Facebook or eBay avoid using client side validation for login?

If they use HTML5 it would just be a "required" word or they could use JS for older browsers that don't support HTML5. So why do they use server side validation?