<style>
.box {
    display: inline-block;
    height: 120px;
    width: 200px;
    margin: 10px;
    border: 1px solid black;
    background-repeat: no-repeat;
}

.linear1 {
    background-image: -webkit-linear-gradient(left, red calc(25%), green calc(50%), blue calc(75%));
    background-image: -moz-linear-gradient(left, red calc(25%), green calc(50%), blue calc(75%));
}

.linear2 {
    /* Green should get shifted to calc(50%) */
    background-image: -webkit-linear-gradient(left, red calc(50%), green calc(30%), blue calc(90%));
    background-image: -moz-linear-gradient(left, red calc(50%), green calc(30%), blue calc(90%));
}

.linear3 {
    background-image: -webkit-linear-gradient(red calc(-50%), green, blue, green, blue calc(150%));
    background-image: -moz-linear-gradient(red calc(-50%), green, blue, green, blue calc(150%));
}

.linear4 {
    background-image: -webkit-linear-gradient(left, red, green calc(100px), blue);
    background-image: -moz-linear-gradient(left, red, green calc(100px), blue);
}

.linear5 {
    background-image: -webkit-linear-gradient(left, red, green calc(120%), blue);
    background-image: -moz-linear-gradient(left, red, green calc(120%), blue);
}

.linear6 {
    background-image: -webkit-linear-gradient(left, red, green calc(50% + 20px), blue);
    background-image: -moz-linear-gradient(left, red, green -moz-calc(50% + 20px), blue);
}
</style>
<body>
    <div class="linear1 box"></div>
    <div class="linear2 box"></div>
    <div class="linear3 box"></div>
    <br>
    <div class="linear4 box"></div>
    <div class="linear5 box"></div>
    <div class="linear6 box"></div>
</body>