Elastic transitions

HTML

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<label>
Your username:
<input value="leaverou"></input>
<span class="callout">
Only letters, numbers, underscores (_) and hyphens (-) allowed!
</span>
</label>

CSS

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/**
* Elastic transitions
*/


input:not(:focus) + .callout:not(:hover) {
transform: scale(0);
transition: .25s transform;
}

.callout {
transition: .5s cubic-bezier(.25,.1,.3,1.5) transform;
transform-origin: 1.4em -.4em;
}

/* Styling */
body {
padding: 1.5em;
font: 200%/1.6 Baskerville;
}

input {
display: block;
padding: 0 .4em;
font: inherit;
}

.callout {
position: absolute;
max-width: 14em;
padding: .6em .8em;
border-radius: .3em;
margin: .3em 0 0 -.2em;
background: #fed;
border: 1px solid rgba(0,0,0,.3);
box-shadow: .05em .2em .6em rgba(0,0,0,.2);
font-size: 75%;
}

.callout:before {
content: "";
position: absolute;
top: -.4em;
left: 1em;
padding: .35em;
background: inherit;
border: inherit;
border-right: 0;
border-bottom: 0;
transform: rotate(45deg);
}