Include details-shim.js anywhere in your page, it runs on load.
Include details-shim.css (or its contents) in your styles.
Tested successfully in FireFox Win/Mac/Lin.
Non-interfering in Chrome Win/Mac/Lin, which supports details/summary.
Non-interfering in Safari Win/Mac, which supports details/summary.
Tested successfully in Internet Explorer 9.
Tested mostly successfully in Internet Explorer 8 with html5shiv.
(IE doesn't like the unicode css :before content)
Tested unsuccessfully in IE7-, considered unsupported
FireFox changes <details open> to <details open="">
Safari supports <details> natively, but treats
Released under MIT license, see included LICENSE.txt or http://opensource.org/licenses/MIT