You can start with checking the HTML source code of the pages of your website for compliance with W3C standards. To do this, go to the page validator (http://validator.w3.org) and in the Address field enter the URL of the page being validated. This is enough, but you can define additional check parameters - they appear when you click the link More Options. To start the verification process click the Check button. After a few seconds the validator will report on his analysis of the code page. It will either be a greeting with a perfectly valid HTML code, or the message about the number of compliance issues. For each error will be indicated, where it is given its description.
The same validator is to check for errors in CSS code. His address - http://jigsaw.w3.org/css-validator. The procedure differs from validation of HTML code that this page has a Russian version. Go to site validator enter the address of the page containing the check code, and, if desired, specify additional validation options in opening them by clicking "advanced features". Then click "Check out". If CSS descriptions are contained in individual plug-in you file, not hardcoded page, specify the address of this file. The test result will also contain either a list of errors with the description, or congratulation with the excellent quality of CSS code.
The validators on the W3C site includes a link checker that checks the availability of links leading "nowhere", i.e. on a page that no longer exists. His address - http://validator.w3.org/checklink. The procedure of verification is as simple as the previous two - go to the website validator, enter the URL of the check page, if necessary, specify additional settings, and then click Check. In the case of broken links validator will list them with indication of the code and their text description.