CSS (Cascading style sheets) enable you to use a common design format across your web site with greater control over all the HTML elements.
For example you can declare that all the main headings are a certain font, size and color: then you simply change that one declaration if you want to change the format of all the headings on your site.
Some time ago I took the plunge and rewrote a huge chunk of my site supertips.com (some 70 pages) to use CSS.
One immediate visual benefit was the smooth flow as I clicked from page to page and saw the same layout, color and format for headings and text compared to the previous assortment of different size headings and sub-headings.
The HTML code is now so simple that I no longer use a web design program to make changes to my pages – I just use a text editor (NoteTab)
Here are two indispensable free tools which helped me
1. W3C CSS Validation Service
Includes checks for invalid CSS which works fine in Internet Explorer but not in Netscape. Click here
2. TidyGui
This cleans up your code and also adds the closing tags for the elements.