Tooltips usually are on links. For example here on HN, hover over "11 minutes ago" in the comment header and it shows the time. Could this be made available to mobile users? Seems tricky. My first thought would be to show it when the user taps and holds. But that also triggers the browsers default context menu for links. So it would make a murky experience.

This is about the best and most intuitive solution you could come up with to this problem. Sadly I believe the Samsung Galaxy S4 was the only phone that supported this without an S Pen so you could simply hover over the screen with your finger. I wish it was a standard feature on all phones.

I think most of the current displays can detect a finger hovering over the display but not necessarily touching it.

It might be a complete disaster though, just wondering how that info could be used.

The first doesn't exclude the second. That's the way how https://xkcd.com/ is doing it. It just shows both.