You can check it using an online tool at http://html5test.com/ Just open up your browser (the one you want to test) and go to the site. It will tell you what is the score. To know how your browser did, divide the score by 3 (ie. If your score is 150, then your browser got 150/3= 50% marks).
Web is huge- about 1.7 billion people use it. You may ask, while talking about the size of web, why I have used the number of people instead of number of website? The answer lies in the modern web technologies. Probably I would use the number of websites, if it were the era of web 1.0 technologies only. But web 2.0 has given power to people. There were few good reasons for that. Web 1.0 was becoming so huge, it became almost impossible for the administrators to keep all the information updated. There were too less help and too much information. Thanks to web 2.0, which enabled any user to update those information.