Yes, even without JS a website can tell what size your browser window is. One way would be to use a large amount of media queries like so: @media (min-widt

Yes, even without JS a website can tell what size your browser window is. One way would be to use a large amount of media queries like so: @media (min-width: 1000px) { #tester-1000 { background-image: url("1000.png"); } } You could also imagine a website first using ~15 queries to know what the window width is upto 100px, and then provide coarser media queries on the next page load.