css transition
<style>
div {
background-color: blue;
padding: 40px;
color: white;
cursor: pointer;
-webkit-transition: all 0.5s 0s ease;
-moz-transition: all 0.5s 0s ease;
-o-transition: all 0.5s 0s ease;
transition: all 0.5s 0s ease;
}
/*transition: property duration timing-function delay|initial|inherit;*/
.hidden {
visibility: visible;
opacity: 0.1;
max-width: 12%;
}
.hidden:hover {
visibility: visible;
opacity: 1;
max-width: 100%;
}
</style>
<div class="hidden">HOVER ME!</div>