
  mask-image: linear-gradient(
    to right top, 
    rgba(0,0,0,1) 40%, rgba(0,0,0,0) 60%
  );


  mask-image: url(IMAGE.png);


  mask-image: url(IMAGE.svg);


  mask-image: url(data:image/svg+xml,...);


  mask-image: url(#svg-mask);

~~Chrome~~ , ~~Safari~~


  mask-image: 
    radial-gradient(circle at center,
      rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%),
    linear-gradient(to right top,
      rgba(0,0,0,1), rgba(0,0,0,0) 75%);


  mask-image: radial-gradient(
      circle at center,
      rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%);
  mask-clip: content-box;


  mask-image: radial-gradient(
    circle at center,
    rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%); 
  mask-size: 50% 50%;


  mask-image: radial-gradient(
    circle at center,
    rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%);  
  mask-size: 50% 50%;
  mask-repeat: no-repeat;


  mask-image: radial-gradient(
    circle at center,
    rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%); 
  mask-size: 50% 50%;
  mask-position: 50% 50%;


  mask-image: radial-gradient(
    circle at center,
    rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%); 
  mask-size: 50% 50%;
  mask-origin: content-box;


  mask-image: 
    radial-gradient(circle at center,
      rgba(0,0,0,1) 70%, rgba(0,0,0,0) 71%),
    linear-gradient(to right top,
      rgba(0,0,0,1), rgba(0,0,0,0) 75%); 
  mask-composite: intersect;

~~Chrome~~ , ~~Safari~~


  mask-image: 
    linear-gradient(to right top,
      rgba(0,0,0,1) 40%, rgba(255,255,255,1) 60%); 
  mask-mode: luminance;

~~Chrome~~ , ~~Safari~~

Test page of mask properties