WEBP Support detector. Adding class to html element.

web-support-detector.js

!function () {
    function getWebpSupport() {
        var canvas = document.createElement('canvas');
        return canvas && canvas.getContext && canvas.toDataURL('image/webp').indexOf('data:image/webp') == 0
    }

    var webpSupport

    if (document.cookie.indexOf('webp') == -1) {
        webpSupport = getWebpSupport() && 1 || 0
        document.cookie = 'webp=' + webpSupport + '; path=/'
    }
    else {
        var webpMatch = document.cookie.match(/webp=(\d)/)
        webpSupport = webpMatch && +webpMatch[1] || 0
    }

    if (webpSupport) {
        document.documentElement.className += ' webp-supports'
    }
}()