Set a grey to white gradient background . Give it a small background-size (by default, this repeats to cover the entire element). This is the pattern part (small, repeating pattern parts). We could also use thin blurry stripes - linear-gradient , rays - repeating-conic-gradient , rings - repeating-radial-gradient . See this demo for examples.

Add another grey to white gradient layer covering the entire element. This is the map part . Where the map is darker, it will make the pattern thicker . This could even be an actual image - url(myimg.jpg) .

multiply the two background layers. The blurry dots pattern looks now darker where the map is darker than where the map is lighter. multiply literally means multiply corresponding decimal representations of percentage RGB channel values (for example, 50% grey has all channel values used in computations) for each pair of corresponding pixels in the two layers.

contrast() with values >1 pushes darker greys (that is, darker than a 50% grey, which is dead in the middle between black and white) towards black and the other lighter greys towards white. A large enough value pushes them all the way to either black or white. The darker blurry dots have more of the blur around them pushed to black than the lighter ones.