impost-pseudo-style.js

var setPseudoStyle = (function() {
	document.head.appendChild(document.createElement('style'));
	style = document.styleSheets[document.styleSheets.length-1];

	return function setPseudoStyle(elm, pseudo, text){
		var id = elm.dataset.psudoLinkToStyle
		if(void 0 === id){
			id = style.rules.length;
			style.addRule( '[data-psudo-link-to-style="'+id+'"]:'+pseudo, '', id);
			elm.dataset.psudoLinkToStyle = id;
		}
		style.rules[id].style = text
	}
})()

credo funzioni così

setPseudoStyle(myElm, "before", "Hello")