Re: [w3ctag/design-reviews] Design Review: Speculation Rules (Prefetch) (Issue #721)

I looked at this briefly during a breakout today, but we ran out of time before discussing with the rest of the group, so these are just my own thoughts, and do not necessarily represent TAG consensus (yet):

- I do agree that the current syntax for prefetch/prerender is clumsy and tedious, and the Quicklink example is quite compelling. Although, with the current syntax, a library would still be needed to do what Quicklink does, since there is no criteria for "is this link in the user's viewport?"
- Any syntax for speculatively prefetching a lot of stuff brings up concerns about sustainability etc. It's a lot of wasted bandwidth, which [for some users may be prohibitively expensive](https://whatdoesmysitecost.com/). If we are to make this a whole lot easier, which will lead to even more wasted bandwidth, there should be a way for users to opt out.
- I agree it's weird to use a different language (JSON) to essentially annotate HTML elements but I also agree that extending HTML with these annotations would be clumsy.
- On syntax: Cramming the entire logic for a conditional in a property name is not very extensible. E.g. `if_not_selector_matches` is essentially a microsyntax for doing negation and specifying what this criteria is going to match on (selector, href, etc). These could be entirely independent if the conditionals are an array of object literals, with one object literal per conditional. This would also allow for additional matching metadata in the future, which you may need for other criteria. E.g. if a proximity to cursor criteria is introduced, you may want to specify distance, velocity etc, if a viewport criteria is introduced you may want to specify offset etc. With the current syntax, each criteria only takes a single argument.

> A rule may include a score between 0.0 and 1.0 (inclusive), defaulting to 0.5, which is a hint about how likely the user is to navigate to the URL. It is expected that UAs will treat this monotonically (i.e., all else equal, increasing the score associated with a rule will make the UA speculate no less than before for that URL, and decreasing the score will not make the UA speculate where it previously did not). However, the user agent may select a link with a lower author-assigned score than another if its heuristics suggest it is a better choice.

I would imagine the likelihood of a link being clicked might change throughout the user's interaction, so I wonder if this would make more sense as an `<a>` attribute? That way it can be updated by script to account for things like is it in the viewport, is the cursor close to it etc or any other arbitrary thing that makes it more or less likely to be clicked.

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