<!DOCTYPE html>
<html>
<script>
internals.settings.setCSSVariablesEnabled(true);
</script>
<style>
div {
  border-color: green;
  border-style: solid;
  border-width: -webkit-calc(--webkit-var(a) + 1px);
  -webkit-var-a: 5px;
}
</style>
<div>This text should have the default border width</div>
</html>