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
Can be used: WDG HTML Validator, W3C Validation Service or any other from this list.
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 browser. Most browsers used on the Internet are: Internet Explorer, Mozilla Firefox and Opera, Safari and Google Chrome
Viewing pages can be done manually by 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 website and wait for a little (in case the images are not showing up, try to refresh the page)
If you wold like to check HTML version you should use Docktype detection. You can read here about how to check HTML version. Html uses specifyc Doctype declarations depending on HTML version.
Good Luck !