<svg xmlns="http://www.w3.org/2000/svg">
<!-- References to external mask resources are not supported. This mask should not be used -->
    <mask id="mask" mask-type="luminance">
        <rect width="200" height="200" fill="black"/>
        <rect x="50" y="50" width="100" height="100" fill="white"/>
    </mask>
<rect width="200" height="200" fill="green" mask="url(resources/mask-resource.svg#mask)"/>
</svg>