Headless-Render-API.com renders any single page of a JavaScript app into an HTML string, and then sends it as a response to the client. From that point on, the client JavaScript app (React, Angular, Ember, Preact, Vue etc...) takes control. There are 2 issues to making this server to client transition graceful:
Either of those issues can result in a "flash" or "blink" where the app re-renders to a zero/empty state before fully rendering again.