Is the strict reliance on W3C CSS validator justifiable or an unnecessary impedance in colleges?

Ive been coding html/css/js/php and various other langs for a couple decades now. Recently applied for a course to get a diploma in Python development.

On my first submission I had to do a "simple" HTML/CSS project. A piece of cake I thought. Turns out I can't even use simple nested css selectors cause W3C validator hans't been updated in donkey years!

Why do colleges keep religiously relaying on an outdated piece of software? Is this justified? Did you college ever make an exception?