A lot of people wanted Steve Klabnik to elaborate on this from a recent post on WebAssembly…

Some have compared WebAssembly to Java applets; in some ways, they’re very right, but in some ways, they’re very wrong. Eventually I’ll write a post about the wrong, but for now, the right: in some sense, WebAssembly is a different way of accomplishing what the JVM set out to do: it’s a common virtual machine that can be used to build very cross-platform software.

Here’s a great take away if all you want is a tldr…

Java Applets and Flash were security nightmares. WebAssembly, on the other hand, piggybacks on the JavaScript VM. All of the effort going into creating its sandbox also applies to Wasm.