Passed validation with wrong markup....

Hi, 
Doing a webcourse, and I was NOT able to hop over when clicking the links on a page by one of the students.
I got nowhere in Opera AND FF (page not found).
IE had no problem with it and takes me to there.
When looking in the source I saw that the links had a BACKSLASH in it, instead of a SLASH.
I'd say that is not right?
So why is it that the w3c validator gives a passed validation for a sloppy code? 

Let me know, okay?
Thanks
sherill