<!DOCTYPE html>
<style>
body {
margin: 0;
}
#big-black-box {
position: absolute;
background-color: black;
-webkit-background-clip: text;
width: 100%;
bottom: 0;
}
#mask {
color: transparent;
}
</style>
<div id="big-black-box">
<span id="mask">Background mask</span>
</div>