As of v0.20.2 we're featuring Webkit vendor prefix -webkit-linear-gradient in .oo-ui-background-image-svg-internal() mixin.
We could probably save a few KBs even after gzipping by removing webkit vendor prefix. It is currently still necessary for Android up to 4.3, Safari 6, Chrome 25 and UCBrowser with about 10% share (9.4% UCBrowser 11 alone). That should be on our radar for the next 12-18 months.
We've got PNG fallbacks, so we are not completely loosing the interface abilities.