Web site Validation is also very important for web designers.
What is meant by validation?
A group known as the World Wide Web Consortium (W3C) develop a set of standards for html and other web languages. If your web site complies with all of their standards, your site is considered valid. If your site doesn't meet the standard, you are invalid. If you page is not valid, it may load slowly and incorrectly. A few small errors wont cause you site to display horrible, but it just looks nicer when you go the extra mile.
Why validate?
Validation is important for 3 reasons Browsers display Web pages and trying to display them even if they're not legal HTML. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal code; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a very poor web page.
That's why you want to make sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validaters.
The second reason is speed: Validation can increase the speed of loading the documents. If your full of errors, It will take longer.
The final reason is your honor as a coder. No good coder wants to put out something riddled with errors. The extra time spent validating shows your visitors that you care enough to make a good product.
Validation is important to having a web developed site.
A free online validation service can be found at: http://www.htmlvalidator.com/lite/
The CSE HTML Validator can also be found at http://www.htmlvalidator.com/lite/. They have a free version of their validator that you can download and use to make sure all your web pages are valid. I use it and it works well for me.