<!DOCTYPE html>
<html>
<script>
if (window.internals)
    internals.settings.setCSSVariablesEnabled(true);
</script>
<style>
body {
    border: 1px solid green;
    border-top-left-radius: -webkit-var(a) -webkit-var(b);
    -webkit-var-a: 3px;
    -webkit-var-b: 6px;
}
</style>
This text should be surrounded by a green border with a rounded top left corner.
</html>