[css-backgrounds] [css-images] Interop for sizing gradients with only width

This following code acts differently in Chrome and SeaMonkey(Gecko).
Which is correct?

Chrome gives the same result as if the background-size specified 33% for
the height. SeaMonkey thinks it should have a height of 100%.

div {
    width: 160px;
    background-repeat: repeat-x;
    background-size: 33%;
    height: 400px;
    background-image: linear-gradient(red, red);
    border: black solid 1px;
}

Google Chrome: 57.0.2987.133 (Official Build) (32-bit)
ec33cd0c06881d919ac0de419d829ad914e0be8f-refs/branch-heads/2987@{#887}
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0
SeaMonkey/2.46