function(a){                            // Supply a valid selector (e.g '*' for all)
  [].forEach.call(                      // Convert Nodelist -> Array
    document.querySelectorAll(a),       // Query for the selector (could be shortened to $() as Chrome & FF expose it in DevTools)
    function(b){                  
      b.style.outline = "1px solid #" +  // Set the selector outline 
      (+~~(Math.random()*(1<<24)))     // Pick a valid random CSS hex color
      .toString(16)});                  // Convert to a base 16 number. BOOM.
}