What is CSS Exactly?
Cascading Style Sheets (or CSS) is recognized as the recommended way to maintain the presentational level of a website document.
The main and utmost important benefit of using CSS over presentational markup imbedded in HTML is that all styling can be kept separated from the content of the document. You can create a single CSS file and attach it to your web pages for easy maintenance.
For instance, if you had a website that consisted of 1000 pages and you wanted to change the text color of your <H1> tag, you could easily update all 1000 pages instantly by modifying one CSS file. If your presentational markup was imbedded into each HTML web page, you would have to edit 1000 pages if your website look was to remain consistent throughout the entire site.
Simply attaching one or even a few external CSS files to your web document offers a great number of benefits. It would be difficult to overstate just how beneficial using CCS is but in the simplest of terms: it is obviously much more efficient editing (or maintaining) one file instead of 100, 1000, or 10,000 HTML documents.
Sometimes with smaller sites, some designers may not recognize the benefit but I highly recommend getting into the practice on sites of any size. I can speak for experience… it save huge amounts of time.