(function nojs () {
    'use strict';

    var docEl = document.documentElement;
    var classRE = new RegExp('(^|\\s)no-js(\\s|$)');
    var className = docEl.className;

    docEl.className = className.replace(classRE, '$1js$2');
})();

(function nojs (html) {
    'use strict';

    html.className = html.className.replace(/\bno-js\b/, '');
})(document.documentElement);

Update no-js-class.js.