Now a day answer like that is very easy find just by using a search engine like Google as irishman suggested above.
I usually go to sites like wikipedia.org for general answer:
For a more detail answer then I look for sites that are focusing on the topic of interest. For example, for CSS and web design, there is a section for that in about.com:
Members here may suggest other valuable resources as well as answer to your question directly. There is a bunch of members here that are very knowledgeable of topic like this.