JSFiddle - React, Tailwind, and code Playground

HTML

<div class="background">
    This is some text blah
        </div>
</div>

CSS

body {
    background-color: blue;
}
#element {
  position: relative;
  width: 200px;
   height: 200px;
   border: 1px solid black; 
}

.background {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -1;
    background-color: grey;
    opacity: 0.3;
    filter: alpha(opacity = 30);
}