<script>
internals.settings.setCSSVariablesEnabled(true);
</script>
<style>
div {
    -webkit-var-myborder: 10px solid green;
    border: 10px dashed red;
    border: -webkit-var(myborder);
}
</style>
<div>
This text should be surrounded by a green border. There should not be a red border.
</div>