As it currently stands, you can write JavaScript, and it will run quite happily in any browser. Given that you're reading a book on C#, you'll be (at least vaguely) familiar with the JavaScript syntax. In this section, I'll try to outline the reasons why you may choose to use WebAssembly over the possible alternatives.
Due to the nature of this comparison, it inevitably contains some opinion. For example, I'm about to extol the virtues of statically typed languages over dynamic ones. If you know and are happy with using JavaScript, you may wish to skip the sections that don't interest you.