fix: client prerender fallback

[rossrobino]

Changes

Prerendering with the Speculation Rules API can fail---for example, certain Chrome extensions like LastPass can block prerendering, or browsers like Arc have disabled the feature. Currently in this case, neither prerendering or prefetching occurs.

Adding a link tag in addition to the speculation rules script does not create an extra request. This change will append the speculation rules script, in addition to prefetching with link or fetch to ensure at least prefetching will occur.

Testing

No modifications needed.

Docs

/cc @withastro/maintainers-docs for feedback!

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[bluwy]

If those extensions block prerendering, wouldn't they also block prefetching? And if Arc disables the feature, wouldn't HTMLScriptElement.supports('speculationrules') be false? I'm a bit worried about this approach, ideally we trust prerendering as the way for snappier navigations in the future, and wouldn't fallback to the old approach again.

[rossrobino]

Thanks @bluwy, agreed that it doesn't seem like it should be necessary if the API is supported.

Discussing this issue on the Speculation Rules spec repo to ensure the best approach is implemented.

[bluwy]

Thanks for the link (and hooking me on the email conversation). Yeah I think it's nicer if the API can fallback to prefetching if prerender is blocked.

I've also found adding the url to both prefetch and prerender works as well.

Seems like Domenic also recommended this. If it doesn't cause a double fetch, I like this approach too!

[rossrobino]

Of course! Thanks for looking this over. Domenic suggested still using the Speculation Rules API to prefetch in this case instead of a link tag due to the benefits stated here. I have made those changes, so I think it's all set to fallback using the recommended approach now.