Yes because, unlike most "normal" programmers, I generate HTML via automation most of the time. So any validation errors are just like debugger warnings to me.
...do you separate presentation and content with css?
Yes, as a first principle. The argument associated with this question is right up there with code-behind or code-inline for ASP.NET coders. I don't treat presentation as a second-class citizen.
Do you use div boxes instead of tables?
I use div instead of center.
Have you checked your page at validator.w3.org and corrected some issues until your site was valid ?
Not since the advent of HTML 4.01 and XHTML. I am trying to catch up but the .NET learning curve is dominating the so-called "free" time.
Is browser compatibility and validity important for you ?
No. W3C validation is considerable.
Does the net improve since people mind such topics and try to make valid pages ? Should all pages be valid, or doesn't it matter anyways?
Validation beomes important when standards-based interop' is important. A new product can come on the market and your pre-existing app' might be ready to interact with it (with little modification) as long as there is some standard for interchange. XHTML is
important because most people respect XML far and away above HTML.
My science background led me to using LaTeX way back before 1996 so I have long-standing respect for typography as well as writing effective code. So naturally I want my HTML presentations to rival that of print design.
I know very well that I am in the ignored minority group regarding these ideas.