One of the most exciting things about HTML5 is that it is the main language of the World Wide Web. In other words, just about any device connected to the Web is able to run any games or applications you write in it. That makes the Web a very unique platform.
Doubtlessly, you have heard about or experienced such things as "the console war", where different video game console manufactures fight to win a larger percentage of the market over their competitors. While some people own more than one video game console, the majority of gamers own only one system. So what that means for a game developer is that, in order for their games to sell well, or in other words, in order for their games to be played and enjoyed by the largest possible amount of players, they need to develop the same version of a game for multiple platforms. This is an expensive and very time consuming process. Creating one game is already expensive and time consuming, let alone duplicating all that work for one or two other platforms.
Today, with more and more users world wide using the Internet, you don't have to go through what console video game developers do. As long as your game runs properly in a standard, modern web browser, it will run the same in just about any other browser across the world. In other words, your same code base can be enjoyed by over a billion people because they all run browsers that conform to HTML5. This makes the Web the largest, and greatest platform of all time. And the best part about it—it's totally free to develop for it. You don't need to have a special license or pay royalties to anyone in order to develop games or other applications for the Web.