when i saw code of any other person/interviewee. I know selection of element for a purpose is most important thing.
what i should judge person ability from his code; from a good written, managed, optimized css or how he wrote class and id names?
Or both every time.
Is it bad to be semantic purist all the time, at work? is it not achievable all the time?
Yes. Sometimes the tools doesn't work the way that you would like, and you just have to use something that does work. It's worth a little effort to make the code more semantically correct to make maintenance more effective, but at some point you just can't defend the development cost because the gain in maintenance that it would give is a lot less.
what i should judge person ability from his code; from a good written, managed, optimized css or how he wrote class and id names?
Anyone can have bad habits or incomplete knowledge in some areas. You should rather judge a person on the ability to see the difference between good and bad code when presented with it, and for showing a certain interrest in getting it right.