Isn't ActionScript just a dialect of EcmaScript? I would guess transitioning from that to Javascript would be pretty easy.
Without the nice IDE experience though.
There was an IDE for ActionScript called FDT built on top of Eclipse.
I meant that JavaScript/HTML/CSS has yet to offer anything similar to the Flash development experience.
Well, the Flash IDE itself was a horrible compiler for those who write mostly ActionScript. We used to compile with few open source compilers. I have forgotten the names.