Fill in the background of an HTML element using CSS

Let's assume that you have the following element:

html
<div class = 'Background'>
    Hello, the background will fill automatically in.
</div>

If you want to fill in the background progressively using CSS:

css
.Background{
    animation: fill .4s ease-in-out .4s forwards;
}

@keyframes fill{
    100% {
        box-shadow: inset 0px 0px 0px 30px #42a1ec;
    }
}