elementToCSS3.js

elementToCSS3 = function( e ){
	// optimized for id lookups
	if( e.id ){ return { selector:"#" + e.id, index:0 }; }
	
	var nodeName = e.nodeName.toLowerCase(),
		css3 = { selector: nodeName +  ( e.className ? "." + e.className.split(" ").join(".") : ""), index:0 },
		node = e;
		
	// speedup for parent lookups
	var parent = node.parentNode;
        
	var samekind = parent.getElementsByTagName( nodeName );
	
	// search for the :nth-child
	for( var i = 0; i < samekind.length; i++ ){
		if( samekind[i] == node ){
			css3.index = i;
			break;
		}
	}
        
	
	// create a loopup chain
	while( node.parentNode && node.parentNode !== document.documentElement ){
		parent = node.parentNode;
		// again, optimize for id's
		if( parent.id ){ 
                    css3.selector = "#" + parent.id + " " + css3.selector;
                    return css3;
                }
		css3.selector = parent.nodeName.toLowerCase() + ( parent.className ? "." + parent.className.split(" ").join(".") : "" ) + " " + css3.selector;
		node = parent;
	}
	
	return css3;
};

// returns { selector: "css3 selector", index: "the index of result array where the item is"}

example.js

document.body.onclick = function( e ){
    var element = e.target;
    var css3 = elementToCSS3( element );

    var qsa = document.querySelectorAll( css3.selector );
    console.info( css3 );
    console.log( qsa );
    console.log( element );
    console.log( element == qsa[css3.index] );

};