mix-blend-mode
CSS property that blends an element with what lies behind it using the same modes found in image editing software — multiply, screen, difference and a dozen more.
Two uses are genuinely practical. With difference, text can be placed over anything and remain readable, because it inverts against whatever is underneath rather than having a fixed colour — which solves the problem of a heading over an image you don't control. And multiply over a coloured surface gives a duotone effect on a greyscale image without the image needing editing. The isolation: isolate property limits how far down the blending reaches. The trap is the same as with backdrop-filter: blending happens within a stacking context, and since a transform, an opacity below one, or a filter anywhere in the ancestor chain creates one, the behaviour is frequently surprising.