Part of the design of the feature is that the website doesn't have to specify "on hover" or "on focus", but instead they can just write "eagerness: moderate" (or "conservative", or "eager") and let the browser decide what the exact triggers are. If it turns out that keyboard focus is a useful predictor of whether a link is likely to be clicked, then browsers can add that as a trigger at one of the indicator levels, and websites will automatically get the new behaviour "for free".

Currently, "eagerness: conservative" activates on mouse/pointer down (as opposed to mouse/pointer up, which is when navigation normally happens), which will work for mobile devices as well. And "eagerness: moderate" includes the more conservative triggers, which means that even on devices with no hover functionality, prerendering can still kick in slightly before the navigation occurs.