There's also the Firefox Remote protocol, which is effectively a subset of CDP. Safari/WebKit has many of the same things exposed over the Inspector protocol,

There's also the Firefox Remote protocol, which is effectively a subset of CDP. Safari/WebKit has many of the same things exposed over the Inspector protocol, but it's neither public nor stable. (And is the ultimate origin of CDP, prior to the Blink fork.) There's a variety of things that you can do with an event-based protocol rather than a pull-based one, such as providing the means to intercept network requests or provide interactive debugging.