insertRule - We all know that we can grab a NodeList from a selector (via document.querySelectorAll) and give each of them a style, but what's more efficient is setting that style to a selector (like you do in a stylesheet):
var sheet = (function() {
// Create the <style> tag
var style = document.createElement('style');
// Add a media (and/or media query) here if you'd like!
// style.setAttribute('media', 'screen')
// style.setAttribute('media', 'only screen and (max-width : 1024px)')
// WebKit hack :(
style.appendChild(document.createTextNode(''));
// Add the <style> element to the page
document.head.appendChild(style);
return style.sheet;
})();
// Usage
sheet.insertRule("header { float: left; opacity: 0.8; }", 1);