css/css-flexbox/interactive/flexbox_interactive_flex-basis-transitions.html

<!DOCTYPE html>
<title>flexbox | transitioned flex-basis</title>
<link rel="author" href="http://opera.com" title="Opera Software">
<link rel="help" href="http://www.w3.org/TR/css-flexbox-1/#flex-basis-property">
<meta name="flags" content="interact">
<style>
div {
	width: 20em;
	height: 8em;

	display: flex;
}
span {
	background: yellow;
	width: 8em;
	flex: 1 1 0%;
}
#test, p+div {
	background: #3366cc;
}
div:hover #test {
	transition: flex-basis 4s;
	flex-basis: 100%;
}
p~div {
	margin-right: 1em;
	float: left;
	display: block;
}
p~div span {
	width: auto;
	height: 8em;
	float: left;
}
p~div~div span {
	width: 5em;
}
</style>

<div>
	<span>one</span>
	<span>two</span>
	<span>three</span>
	<span id="test">four</span>
</div>

<p>Verify that the box above looks exactly like the second of the boxes
below. Then hover the box above and leave the mouse there for 4 seconds.
After the specified time, the box above should look exactly like the
first of the boxes below.</p>

<div>
	<span>one</span>
	<span>two</span>
	<span>three</span>
	four
</div>

<div>
	<span>one</span>
	<span>two</span>
	<span>three</span>
	<span id="test">four</span>
</div>