static warmConnections() {
    if (LiteYTEmbed.preconnected) return;

    // The iframe document and most of its subresources come right off youtube.com
    LiteYTEmbed.addPrefetch('preconnect', 'https://www.youtube-nocookie.com');
    // The botguard script is fetched off from google.com
    LiteYTEmbed.addPrefetch('preconnect', 'https://www.google.com');

    // Not certain if these ad related domains are in the critical path. Could verify with domain-specific throttling.
    LiteYTEmbed.addPrefetch('preconnect', 'https://googleads.g.doubleclick.net');
    LiteYTEmbed.addPrefetch('preconnect', 'https://static.doubleclick.net');

    LiteYTEmbed.preconnected = true;
  }
Begin pre-connecting to warm up the iframe load
Since the embed's network requests load within its iframe,
  preload/prefetch'ing them outside the iframe will only cause double-downloads.
So, the best we can do is warm up a few connections to origins that are in the critical path.

Maybe `<link rel=preload as=document>` would work, but it's unsupported: http://crbug.com/593267
But TBH, I don't think it'll happen soon with Site Isolation and split caches adding serious complexity.