If you're following along in the code, four classes are defined, l_and, l_and:visited, l_and_not, l_and_not:visited. l_and:visited and l_and_not are set to bac

If you're following along in the code, four classes are defined, l_and, l_and:visited, l_and_not, l_and_not:visited. l_and:visited and l_and_not are set to background color white, the others black. 512 link 'stacks' are created, each one in a td tag, each one for each possible setting of the 9 sites he checks. Each of the classes get mix-blend-mode: multiply turned on. He then looks for a final image with color white -- that's the bitset of your visited sites, since the multiplies will all give white as the output.