Comment by duffyjp

Years ago before you could do anything this fancy with CSS I experimented with generating 3x2 pixel images server side and then presenting them as base64 encoded pngs in a "scoped" block of CSS to ensure they loaded before the src images. Coincidentally this was the same 3x2 layout as OP did here with CSS. I abandoned it because a 3x2 image scaled up looked terrible, and went with average color instead. This solution looks a lot better visually.

I still do the average color thing today since it's easy to calculate and store server side (I resize the uploaded image to 1x1 px and just record the result as a hex code in the DB).