These are some reasons why you should Validate your code:

1. It Helps Cross-Browser, Cross-Platform and Future Compatibility

Although you may be able to create a web page that appears to work on your favourite browser (whatever that may be), your page may contain HTML errors (or CSS errors) that do not show up with that browser due to an existing quirk or bug. Another person using a different browser that does not share that particular bug will wind up viewing a page that does not show up correctly. It is also possible that later versions of your browser will fix that bug, and your page will be broken when people use the latest incarnation of the browser.

Coding your pages so that it is correct without errors will result in pages that are more likely to work across browsers and platforms (ie, different systems). It is also a form of insurance against future versions of browsers, since all browsers aim towards compliance with the existing HTML and CSS standards.

2. Search Engine Visibility

When there are errors in a web page, browsers typically try to compensate in different ways. Hence some browsers may ignore the broken elements while others make assumptions about what the web designer was trying to achieve. The problem is that when search engines obtain your page and try to parse them for keywords, they will also have to make certain decisions about what to do with the errors. Like browsers, different search engines will probably make different decisions about those errors in the page, resulting in certain parts of your web page (or perhaps even the entire page if your error is early in the page) not being indexed.

The safest way, it is held, is to make sure that your web page validates error-free. That way, there is no dispute about which part of your page should be scanned for keywords and the like.

3. Professionalism

Even if you test your web site with all the various browsers in existence on all the platforms in use (Mac, Linux, Windows, FreeBSD, etc) and find that it works perfectly in all, errors in your site reflect poorly on your skill as a web developer.

The issue is two-fold: firstly, a poorly coded web page reveals that either the web designer does not know his stuff or is a sloppy worker; secondly, it affects his marketability.