I gave up trying to implement Mathias Bynens’ jQuery plugin on my own. I was delighted when I found HTML5 Details Polyfill. Not only does it just work but Anzalone also included the css and a great readme.
The only problem I've had is CloudFlare's experimental Rocket Loader™ feature which tries to asynchronously load js. With Rocker Loader on I get a flash of unrendered content, turning it off relieves the problem.
Thanks Anzalone! I'm gonna try out your other plugins too.