Do you think users really cares about a html and css validated website

[ZiNgA BuRgA]

Users won't care.  But be careful though - I'm one of the few people who have images disabled, so alt tags definitely would help me, but I presume I'm really a small minority (Australian internet sucks, especially if you're screwed by Telstra).

It is ideal to follow a spec though, and I don't think it takes that much extra effort to make your code valid, so why not?  If you're doing a lot of markup it could be helpful, and it increases the likelihood of your stuff working on new browsers that pop up (eg Chrome, or maybe some new mobile browser) without having to make many amendments.