As everyone knows, validity of source code plays important role in a web development. Improperly nested elements, unclosed tags, unrecognized parameters may affect web page's display in some browsers. But this is not one and only negative influence of source code errors. Another important consequence of invalid (X)HTML or CSS source code is degradation in the eyes of search crawlers. Most search engine algorithms penalize web pages with errorneous code.
To avoid decreasing of your visitors or degrading your page rank, it is strongly recommended to valid the source code against World Wide Web Consorcium (W3C) standards. These on-line tools can discover all errors in your source code and offer explanation of them as well as remedial action to be fully in compliance with recommended standards.
All you have to do is go there, submit the URL of the page which should be checked for errors in source code and then follow instructions returned by W3C validation service. What a low price for correct (X)HTML/CSS source code which leads to better search engine rankings and more satisfied visitors!