Building C++ to target Javascript (e.g., asm.js) and execute C++ in the browser is becoming quite the popular indoor sport. Here's a current presentation and experience report:

C++ on the Web: Run your big 3D Game in the browser! (slides)

by Andre Weissflog
Head of Development, Berlin
Bigpoint GmbH

My presentation about porting large C/C++ code bases to the browser (emscripten, flascc, Google Native Client)

From the Wrap-up slide:

• You can run big C/C++ code bases ("a million lines of code") in the browser.
• Javascript is already fast enough for many types of games.
• Massive performance improvements happening right now (better code generation, JS engines better at running generated code, asm.js...)
• ...