<!DOCTYPE html>
<html>
<style>
.box {
width: 200px;
height: 50px;
display: inline-block;
zoom: 2;
}
.rgb-background {
background: linear-gradient(
to right,
red,
red 25%,
green 25%,
green 75%,
blue 75%
);
</style>
<body>
<div class="box rgb-background"></div>
</body>
</html>