Good question. Preload that actually renders the documents includes running enough javascript and parsing enough CSS that the initial document will render. With

Good question. Preload that actually renders the documents includes running enough javascript and parsing enough CSS that the initial document will render. With a non-constrained document, that can include fetching resources not on Google's servers and which can peg the CPU, causing jank. If you do constrain what can run in your preload to avoid these, you end up designing AMP.