Now try Google's sweetheart project https://www.ampproject.org/ or any amp-powered site. They contain this gem of a boilerplate hiding page content for 8 seconds or until whatever script is finally loaded that overrides it:

    body {
      animation: -amp-start 8s steps(1,end) 0s 1 normal both; 
    }
    @keyframes -amp-start {
      from { visibility: hidden; }
      to { visibility: visible; }
    }