<!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>