Preconnect/dns-prefetch resource hint against web.dev best practices

[schlessera]

I think this combined preconnect and dns-prefetch resource hint is not always doing what it is supposed to do depending on the browser:

amp-toolbox/packages/optimizer/lib/transformers/GoogleFontsPreconnect.js

Line 55 in 84cb305

rel: 'dns-prefetch preconnect',

As far as I can tell, the dns-prefetch is meant to be the fallback in case the preconnect is not supported (as the preconnect would include a DNS resolution). However, the dns-prefetch is mentioned first, which seems odd.

Also, according to web.dev, best practice is to use two separate hints with the dns-prefetch as the second hint serving as a fallback, as using a combined hint will break in Safari:
https://web.dev/preconnect-and-dns-prefetch/#resolve-domain-name-early-with-reldns-prefetch

[schlessera]

Related issue in Go optimizer: ampproject/amppackager#489