Version: 52.0.2743.116 (64-bit)
OS: OSX El Capitan  V10.11.6 (15G31)

What steps will reproduce the problem?
(1) The following style sheet:
<style>
  :root {
    --foo: {
       background-image: url(//app-layout-assets.appspot.com/assets/bg1.jpg);

      /* This does work: background-image: url(https://app-layout-assets.appspot.com/assets/bg1.jpg); */
    };
  }

  body {
    @apply --foo;
  }

</style>

Repro: http://jsbin.com/ruvoki/edit?html,output


What is the expected output?

The background-image property should work.

What do you see instead?

The background-image property is ignored unless the scheme is specified.