iOS/iPhones lack the CSS cursor ::selection pseudo-element. I believe macOS, iPadOS, and possibly visionOS support the events. Considering visionOS seems to be very similar to iPadOS I am assuming visionOS supports it as well. The ::selection CSS pseudo-element applies rules to the portion of a document that has been highlighted (e.g., selected with the mouse or another pointing device) by the user.
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iOS, iPadOS, and visionOS lack support for the CSS ::selection pseudo-element. Here is a demo: https://quirksmode.org/css/selectors/selection.html
I have also run into this issue. I suspect there might be some complications due to the fact that iOS shows selection handles on the left and right edges of the selected text, but I'd very much like to be able to style the ::selection pseudo-element in Safari on iOS, including background color, text color, and/or underline style(s), as I can in Safari on macOS.