insert-css-to-head-for-google-optimize.js

/**
 * Function adds style element to the bottom of the head element of the page
 * @param  {string} code Any string of css code
 * @return {undefined}
 */
function insertCssToHead( code ) {
    var style = document.createElement('style');
    style.type = 'text/css';

    if (style.styleSheet) {
        // IE
        style.styleSheet.cssText = code;
    } else {
        // Other browsers
        style.innerHTML = code;
    }

    document.getElementsByTagName('head')[0].appendChild( style );
}

// Example usage;
// insertCssToHead('body{ background-color: red; }');