Is it possible to know everything and every concepts in computer science or should I focus my career in one particular area of cs... ??!

You will never know everything. There is a lot to learn and study and a lot of new stuff are coming out very fast. I guess this applies to almost all domains, not only software development.

First priority, I say, is to focus on what you are doing on your career and get the best out of it. If you like html and css, become the best on writing them and learn how to do professional websites.

Secondly you should study other topics to expand your knowledge, so you have more options to choose from.
It's good that you know web development very well, but it would be awesome if you know mobile development as well. (just some examples)