Prerendering a page per device, and the cache key that has to agree

A marketing page with no per visitor content should be built once and handed out as a file, which is what prerendering means here: the build writes the HTML, and the server answers with it instead of rendering again. The page I had to do this to was not quite static. Its footer is an accordion on a phone and a grid on a laptop, and a few sections below it differ the same way. Two rendered…