A polyfill to add window.CSS.supports support to browsers that support @supports in CSS, but lacks the JavaScript API. It uses window.supportsCSS if available.
window.CSS.supports is the proposed object defined in the CSS Conditional Rules Module Level 3 specification. Opera added support for window.supportsCSS() in version 12.10 of the browser. Though @support is enabled in Firefox 17.0 behind a flag, the JavaScript API is not yet available.
Other browsers have not yet added support (check current support tables).