mask-mode

The mask-mode CSS property sets whether the mask reference defined by mask-image is treated as a luminance or alpha mask.

/* Keyword values */
mask-mode: alpha;
mask-mode: luminance;
mask-mode: match-source;

/* Multiple values */
mask-mode: alpha, match-source;

/* Global values */
mask-mode: inherit;
mask-mode: initial;
mask-mode: revert;
mask-mode: unset;

The mask-mode property is specified as one or more of the keyword values listed below, separated by commas.

alpha

This keyword indicates that the transparency (alpha channel) values of the mask layer image should be used as the mask values.

luminance

This keyword indicates that the luminance values of the mask layer image should be used as the mask values.

match-source

If the mask-image property is of type <mask-source>, the luminance values of the mask layer image should be used as the mask values.

If it is of type <image>, the alpha values of the mask layer image should be used as the mask values.

<masking-mode>#
where 
<masking-mode> = alpha | luminance | match-source

• Clipping and Masking in CSS