If you lay your eyes on an electronic device, chances are that there is a browser running on it! You probably have more than one installed on each of your PCs and some more running on your portable devices. If you want to distribute your games to a wide audience for a minimal cost of entry, making it run in the browser makes a lot of sense.
Flash was for a long time the go-to platform for games in browsers, but it has been losing speed in the last few years. There are many reasons for this and there have been countless arguments about whether this is a good thing or not. There is, however, a consensus on the fact that you can now make games run in the browser without plugins at a reasonable speed.
This book will focus on 2D games as they are the ones that run well on current browsers and the features they depend on are standardized. This means that an update of the browser shouldn't break your games and that for the most part you don't have to worry too much...