Validating HTML is one important step to ensure that your webpage is constructed of valid and well-formed HTML. This will give you some assurance that your page will work on many web browsers; however, the only way to be certain that your webpage is widely compatible is to test it on a variety of platforms and browsers.
Try validating a few of the pages you have created in this course; also try downloading the HTML of a website that you go to frequent and try validating it. Also remember to check the compatibility of your code by opening it in a variety of web-browsers - minimally, the current versions of Firefox, Chrome, Opera, and Internet Explorer on a Windows machine and Firefox, Chrome, Opera, and Safari on a Mac OS/X machine. Ideally, you would also check your code's compatibility on older browsers and on browsers on GNU/Linux, iOS, Android, Windows Mobile, and RIM/Blackberry platforms before going live with a high profile website. For a personal website, this level of attention to detail may not be warranted, however. Alternatively, there are a number of webtools that will test a site for cross-browser compatibility once the site is already live.