Good to know. Yeah, it's pretty easy to add 0, so I'll do that soon. For the kakuro backgrounds, it's pure CSS, no images. The diagonal split in the clue cells is a single linear-gradient hairline, not a tile or PNG:

.kakuro-clue { background-image: linear-gradient(to top right, transparent calc(50% - 0.5px), rgb(148 163 184) calc(50% - 0.5px), rgb(148 163 184) calc(50% + 0.5px), transparent calc(50% + 0.5px)); }

The site uses Tailwind CSS for the styling generally.