css/css-cascade/layer-property-override.html

<!DOCTYPE html>
<title>Resolving @property name conflicts with cascade layers</title>
<link rel="help" href="https://drafts.csswg.org/css-cascade-5/#layering">
<link rel="author" href="mailto:xiaochengh@chromium.org">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
#target, #reference {
  width: 100px;
  height: 100px;
}

#reference {
  background-color: green;
}
</style>

<div id="target"></div>
<div id="reference"></div>

<script>
// In all tests, background color of #target should be green, same as #reference

const testCases = [
  {
    title: '@property unlayered overrides layered',
    style: `
      #target {
        background-color: var(--foo);
      }

      @property --foo {
        syntax: '<color>';
        inherits: false;
        initial-value: green;
      }

      @layer {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: red;
        }
      }
   `
  },

  {
    title: '@property override between layers',
    style: `
      @layer base, override;

      #target {
        background-color: var(--foo);
      }

      @layer override {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: green;
        }
      }

      @layer base {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: red;
        }
      }
    `
  },

  {
    title: '@property override update with appended sheet 1',
    style: `
      @layer base, override;

      #target {
        background-color: var(--foo);
      }

      @layer override {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: green;
        }
      }
    `,
    append: `
      @layer base {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: red;
        }
      }
    `
  },

  {
    title: '@property override update with appended sheet 2',
    style: `
      @layer base, override;

      #target {
        background-color: var(--foo);
      }

      @layer base {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: red;
        }
      }
    `,
    append: `
      @layer override {
        @property --foo {
          syntax: '<color>';
          inherits: false;
          initial-value: green;
        }
      }
    `
  },
];

for (let testCase of testCases) {
  var documentStyle = document.createElement('style');
  documentStyle.appendChild(document.createTextNode(testCase['style']));
  document.head.appendChild(documentStyle);

  var appendedStyle;
  if (testCase['append']) {
    document.body.offsetLeft;  // Force style update
    appendedStyle = document.createElement('style');
    appendedStyle.appendChild(document.createTextNode(testCase['append']));
    document.head.appendChild(appendedStyle);
  }

  test(function () {
    assert_equals(getComputedStyle(target).backgroundColor,
                  getComputedStyle(reference).backgroundColor);
  }, testCase['title']);

  if (appendedStyle)
    appendedStyle.remove();
  documentStyle.remove();
}
</script>