Add dns-prefetch resource hint as fallback for Google Fonts preconnect

[westonruter]

In #37171 the use of the preconnect resource hint was added for core themes that use Google Fonts. However, what was missing was also including the dns-prefetch resource hint. The preconnect resource hint is not supported by older browsers, so including dns-prefetch ensures that they'll also get some performance benefit.

See MDN docs for ​Best Practices that recommend pairing preconnect with dns-prefetch.

[joyously]

It's interesting that caniuse shows that dns-prefetch has 82.5% support, and preconnect has 90.5%. (part of that 90% is very recent)
But it says that Firefox supported preconnect and then dropped it. Is that right?

[westonruter]

Yeah, I'm not entirely clear on that either. But MDN recommends adding both, and dns-prefetch will at least benefit IE11 if Firefox doesn't in fact support any longer. ​According to Andy Davies:

It appears that preconnect doesn't currently work in Firefox even though it's supposed to

See Firefox bug ​1543990 opened 2 years ago.

[dd32]

WordPress 5.7.2 has been released, moving open tickets to 5.7.3

[peterwilsoncc]

This will require new versions of the 2012-2017 default themes.

As this is a long-standing issue, in part caused by a browser bug, I'm going to bump it to the next major release as I don't think it warrants new theme versions alone and complicating the minor release process further.

[hellofromTonya]

Today is 5.8 Beta 1. No progress made in the cycle. As it's bumped multiple times and is a long standing issue (partly due to browser bug), punting to Future Release.

[desrosj]

@westonruter Is this a change still worth making?

The only browser currently supported in Core (and the larger community now that IE11 has reached end of life) ​that does not support preconnect is Firefox. Caniuse says it has not been supported since version 71 and Firefox is currently at version 102.

Firefox also only supports dns-prefetch on HTTP origins. HTTPS is unsupported. 82%+ of active WordPress installs report using HTTPS, so it seems this would only benefit a small subset of sites.

Adding the close suggestion, but if anyone feels strongly this should still be implemented it should be safe to add.