CSS3 RGBA

An element with opacity/alpha value of 1.0 will be fully opaque (visible) while an element with opacity value 0.0 will be the complete opposite, invisible.

<style>
.box_rgba {
background-color: rgba(0, 102, 153, 0.3);
color: rgba(255, 255, 255, 0.8);
font-size:30px;
font-weight:700;
padding: 10px;
}

.box_rgba:hover {
background-color: rgba(0, 102, 153, 1);
color: rgba(255, 255, 255, 1);
}
</style>
<div class=box_rgba>Box RGBA</div>