Sanitize CSS and optionally prepend each selector with another selector to isolate the application of these styles

sanitizecss.js

/**
 * Takes a block of CSS and uses the browser's CSS parsing mechanism to sanitize
 * the content and return only valid style declarations. Optionally you can
 * prepend each selector with another selector to isolate the styles even further.
 * A use case would be accepting user-generated CSS.
 */
function sanitizeCSS(css, selectorPrefix) {
	var style = document.createElement('style')
	  , rules
	  , output = ''
	;
	style.innerHTML = css;
	// need to append the stylesheet to activate the CSSStyleSheet object
	document.head.appendChild(style);
	// save a reference to the sheet
	rules = style.sheet.rules;
	// remove the sheet from the document
	document.head.removeChild(style);
	// loop through all the style blocks and concatenate them
	for (var i = 0; i < rules.length; i++) {
		if (selectorPrefix) {
			output += rules[i].selectorText.split(',').map(function(selector) { return selectorPrefix + ' ' + selector }).join(',');
		} else {
			output += rules[i].selectorText;
		}
		output += ' { ' + rules[i].style.cssText + ' }\n';
	}
	return output;
}