We occasionally see questions about what JS-based sites can do and still be visible in search, so here’s a brief summary for today’s state:
Consider migrating old sites that use this scheme soon. Remember to remove “meta fragment” tags when migrating. Don’t use a “meta fragment” tag if the “escaped fragment” URL doesn’t serve fully rendered content.
When using AMP, the AMP HTML page must be static as required by the spec, but the associated web page can be built using JS/PWA techniques. Remember to use a sitemap file with correct “lastmod” dates for signaling changes on your website.
 Progressive Web Apps : https://developers.google.com/web/progressive-web-apps
 Progressive enhancement:https://en.wikipedia.org/wiki/Progressive_enhancement
 rel=canonical: https://support.google.com/webmasters/answer/139066
 AJAX Crawling scheme: https://developers.google.com/webmasters/ajax-crawling/docs/specification