insert-style-rule.js

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);