In order for your site to rank well, it has to be indexed first and in order for that to happen, search engine spiders visiting your website must be able to follow your HTML code. Sometimes, broken links, missing closing tags, among other commonly made mistakes can keep your site from being indexed properly or not at all. This is where all of your marketing and optimization efforts go to waste. A syntax checker should be part of every webmaster's list of internet marketing tools and part of their system of putting up a website.
What can you expect an HTML syntax checker to find? Sometimes, although everything is displayed in the right order and in the right place, the code might not necessarily be bullet proof. Most web-browsers can recognize small html mistakes and correct them before displaying the page; however, this is not the case for web spiders. A small mistake can cause them to drop your page and prevent it from getting into the index for that particular search engine. The result? Lost traffic.
HTML syntax checkers are really easy to use. Some require you to pop your code into a designated window while others only ask you to specify the URL of the particular page that you want to check. Some tools even let you check code that isn't live yet by specifying the path to the file on your computer. Although the tools tell you what is wrong with the code (some even give you suggestions of what could possibly fix the problem), it is eventually up to you to find it. This is where good debugging skills really pay off.
This is one of the easiest tools to use. All you need to do is type in the address of the live version of a page and you're done. The code of the page is displayed below along with a list of errors and warnings. The line number that is displayed beside the error message is linked up to the spot in the code where the error occurred making it easy and quick to fix. Suggestions of possible fixes are also provided.
W3C MarkUP Validation Service
This is a free service that checks documents like HTML and XHTML for conformance to W3C recommendations and other standards. It gives you the option to check live pages as well as those you haven't uploaded yet by specifying a path to the local file. There is also an Extended Interface available if you want to set any advanced options, and a Extended File Upload Interface if you want to validate uploaded files with advanced options.