<!DOCTYPE html>
<html>
<head>
<style>
div {
    left: 0;
    top: 0;
    position: absolute;
    height: 100px;
}

.mask {
    /* We add an extra pixel to account for rounding errors */
    width: 51px;
    background-color: red;
}

#target {
    width: 100px;
    background-color: black;
    transform: translateX(50px);
}

#right {
    left: 149px;
}
</style>
</head>
<body>
<div id="target"></div>
<div id="left" class="mask"></div>
<div id="right" class="mask"></div>
</body>
</html>