form { clear: both }

button + span.jsreq { display: none }

button:disabled + span.jsreq {
    display: initial;
    animation: jsfadein ease-in 0.1s;
    animation-iteration-count: 1;
    animation-fill-mode: forwards;
}

@keyframes jsfadein {
    0% { opacity: 0; }
    100% { opacity: 1; }
}