Today most of the sites are based on CSS. There are many reasons why CSS have become so popular some of them are easy maintenance of website, clean html code, easy updates, increase download speed, reduce bandwidth and most important improves search ranking ;
You must be wondering how it helps in ranking? Here’s the explanation….

Generally a page of site get index when the spider/bot of search engines crawls content of that page, When a search engine, such as Google, sends a spider to crawl your website, it looks through the code of your webpage they actually read certain elements of your HTML code. Certain HTML code holds more weight than other tags. So it is necessary to have a clean and readable code which is easy to understand by spiders, if there are unnecessary text and tags in your HTML file, spiders will have to struggle to understand that page is related to a particular search keyword. Using CSS you will be able to change the code layout without destroying the actual visual layout and will get an clean html code which will result to better search engine rankings.
eg .

You can easily make the main content of your site to show up above the header or navigation menu in the code of your website; thus will help to show search engine crawlers the importance of your content.

Today major search engines such as: Google, Yahoo and MSN loves to see your content; the text, not the clumsy code. Using CSS you can have pure and clean food for spiders.