Cascading Style Sheets (CSS) provide several SEO benefits for your web site design and developoment.

Website components with the appropriate search keywords and keyword phrase density can be 'seen' by the search engines more effectively and your web pages can be presented to your visitors more effectively.

Your website will operate more efficiently if you incorporate layouts which are compatible with all currently used browsers. The term used to summarize this is to say that the layouts are 'cross-browser compatible'.

The World Wide Web Consortium (W3C) maintains and published many standards for Web functionality.
You should ensure that the layout that you use is compliant with these standards. A standards compliant layout will display correctly in the greatest range of web browsers and will provide efficient, ie faster, website functionality.

Using Cascading Style Sheet (CSS) layout control, rather than table based layout control will provide these benefits and more.

Using CSS for layout

Prior to the widespread adoption of CSS, webpage layout or format was controlled by using multiple, nested HTML Table structures. These were inflexible, and tedious to code. (You can spend a lot of time debugging HTML scripts determining the location of HTML table tags - tedious!)

Advantages of CSS
The search engines 'see' your webpages differently than a human visitor to your website.

If your web design is based on Cascading Style Sheets (CSS), the webmaster will be able to alter the order of the elements in any form or fashion he/she sees fit.

So, CSS based layouts allow you to place your web page content close to the beginning of your web-page file. This is the part of the page files that you want the search engines to locate first as it will be your targetted page content incorporating your targetted keyword phrases. This provides advantages in Search Engine Optimisation (SEO).

As the content part of your web-page file is the first 'seen' by the search engines, the keywords contained within that part will be featured as prominently as possible, instead of prominence being assigned to the header sections of your web-page file, menus and other areas of your page.

Effectively display prominent web-page components  
Prominence can be assigned to components of your web pages by adjusting the order of the components within your web pages.

Using CSS for layout control allows you to decide the ordering of the webpage components within your web page files. It is common practice that the page content be placed first, however the order of the web page code can be rearranged as you (or your SEO specialist) wish.

For example, if a menu contains particular keyword or keyword phrases which you want to feature prominently, the code for that menu element could be placed before the code for the header code and/or any other page element.

Faster Web Page Display
Your visitors will be able to see the content you want to present faster if the content part of your web-page files are loaded into the visitor's browser first, instead of having to wait for other page components like images, menus, general header information, etc, to load first.

Your visitors will be able to view individual pages of your website with minimal delay, while other web page components load around the content. As discussed earlier in this article, the order which these page elements load will be decided by the CSS order and can be controlled for the maximum effect.

In summary, using CSS for your website layout will provide SEO advantages and enable you to provide an effective website viewing experience for your visitors.