Time to first sizing — one photo, three strategies

One ~3MB PNG, loaded three ways. Each panel has its own Run button — click to feel the click-to-dimensions delay for each strategy. Naive <img src> can only tell you dimensions once the full transfer is decoded. Declared width/height attributes are instant but require the author to already know them. prepare() streams the response and parses the format header out of the first ~2KB — dimensions arrive long before the image finishes downloading.