Keep a static "emergency mode" site on S3
Or better yet -- make your entire site static to begin with!  This is how our site works (firebase.com). Our entire site is static content that's generated at deploy time and hosted on a CDN. Dynamic data is loaded asynchronously as needed. If a server were to go down, at least all of the static pieces (which is most of the site) would be unaffected.We use Firebase to power the dynamic portions (obviously), but you can use plain old AJAX requests as well.The age of the dynamically-generated HTML page is coming to an end.