Checking the HTML code of your web pages is done with the help of the HTML Validators
Are pretty practical and easy to use. Pages can be validated by uploading them on the server or by entering the URL address where you can find the page you wish to review
The fact that a link is not functional can often lead to abandonment by the web site user. To verify functionality of the links will use a Link Checker. I recommend the W3C Link Checker or any of this list.
Viewing the browser is also very important. It is possible that from a browser to another to have different show up of a layout, frame, images or any other element of the page
It is recommended that you check the web page in more than one browsers. Most browsers used on the Internet are: Internet Explorer, Mozila Firefox and Opera, Safari and Google Chrome
Viewing pages can be done manually installing multiple browsers on your computer and entering the URL of the web page. A specialized program in checking the design of your page in different browsers can be found at Browsershots.org. Just type the address of your web site and wait a little (in case the images are not showing up, try to refresh the page)
Good Luck !