LayoutTests/fast/images/image-source-assert.html

<style>
* {
    -webkit-mask-image: -webkit-canvas(viewport-mask);
    -webkit-mask-source-type: luminance;
}
</style>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
</script>
<p>The test passes if it does not generate a debug assertion.</p>