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.<p>We use Firebase to power the dynamic portions (obviously), but you can use plain old AJAX requests as well.<p>The age of the dynamically-generated HTML page is coming to an end.