Google has been working on Native Client (NaCl) and Portable Native Client (PNaCl) for a while now. Firefox recently announced and released asm.js. Here's a few random personal thoughts with no particular conclusion
First off a bunch of disclaimers. (1) I work on the Chrome team. (2) I'm speaking for myself, not Google nor the Chrome team. In fact I'm pretty sure some other people on the Chrome team will 100% disagree with some of these opinions. So, these opinions / thoughts are my own and in no way represent Google or the Chrome Team.
Second, let's spell out what NaCl, PNaCl, PPAPI, NPAPI and asm.js are
NaCl is a PPAPI plugin that allows untrusted native code to run in the browser securely. It only supports x86 code. It provides access to PPAPI APIs, sometimes called Pepper APIs.
PNaCl is the portable version of NaCl. Instead of being x86 only it translates LLVM bytecode to the host processor.
NPAPI is the Netscape Plugin API that was developed in the mid 90s to allow browser plugins. It is massively insecure and is the #1 vector for browser exploits. All it defines is how a plugin can talk to a browser. Otherwise the plugin is free to do whatever it wants with your entire machine. An NPAPI plugin can read any file, call any OS function, do any networking or nasty things it wants.
PPAPI is a new plugin standard originally co−developed by Mozilla and Google but apparently Mozilla stopped participating. It provides *cross−platform* out of process APIs for most common system functions. In other words, it lets a plugin do things like audio, graphics, networking and file i/o in a cross platform way. This means a plugin developed for PPAPI should run on every OS and Browser that supports PPAPI. It would have to be recompiled but since the APIs are the same on all platforms it should work with no code changes. This is in contrast to NPAPI plugins which are specific to each OS since they themselves call OS level services.
The combination of PPAPI and PNaCl provides a 100% portable and secure way to run native code in a browser.
The problem now is you'll need to add support for graphics, audio, file i/o, keyboard and mouse input, networking, etc from workers in ways that meet the needs of these special "asm.js workers" and legacy C++ code. Hmmm, sounds like PPAPI.
I don't know if we'll actually get there. It just seems like asm.js and PNaCl are closer than people are admitting.