The following code works in Chrome, but fails in Firefox. This style blinks the success message.
.rainbow {
background-image: -webkit-gradient(linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2), color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22));
background-image: gradient(linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2), color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22));
color: transparent;
-webkit-background-clip: text;
background-clip: text;
}
<div class="rainbow">Success</div>