Set mask-mode to alpha if the mask-image property is an image (an image URL or a gradient). If the mask-image property is an SVG <mask> element, use the <mask-type property of the <mask> element. This is the default setting.
As the mask values, use the luminance values of the mask image.
Use the mask image's alpha values as the mask values.
This property is set to its default value.
This property is inherited from its parent element.