I think this is slightly safer than the @jcubic answer and will not cause the style to be applied to the current document:

function css_sanitize(css) {
    const iframe = document.createElement("iframe");
    iframe.style.display = "none";
    iframe.style.width = "10px"; //make small in case display:none fails
    iframe.style.height = "10px";
    document.body.appendChild(iframe);
    const style = iframe.contentDocument.createElement('style');
    style.innerHTML = css;
    iframe.contentDocument.head.appendChild(style);
    const sheet = style.sheet,
        result = Array.from(style.sheet.cssRules).map(rule => rule.cssText || '').join('\n');
    iframe.remove();
    return result;
}