Используйте opacity
свойство и :hover
псевдокласс.
.box {
opacity: .5;
transition: opacity .5s;
}
.box:hover {
opacity: 1;
}
<div class="box">
<h1>heading</h1>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>
</div>