The most impressive html5 game I've played so far with nice graphics, music, and most importantly pretty responsive feedback is 'Save the day' :
https://turbulenz.com/games/save-the-day/play
The first time I played it, I had assumed it was a Flash game, and not html5, based on the quality of it.
It forced me to download a DMG containing "TurbulenzEngineInstaller.app" which itself contains "npturbulenz.plugin" and an "install_script" that installs that plugin into "/Library/Internet\ Plug-Ins/".
About as good as you can get plugin installation to be, I guess, but it's not what I'd expect from an HTML5 game.
The site offers to install a plugin as a fallback for browsers that don't appear to offer the required HTML5 APIs (mainly WebGL). This allows the JavaScript/WebGL games to run on a wider range of browsers (particularly IE and when required APIs are locked behind a developer switch.)
The first time I played it, I had assumed it was a Flash game, and not html5, based on the quality of it.