move style changes for opening/closing details into UA sheet rather than being style attribute manipulation

[dbaron]

What is the issue with the HTML Standard?

In #10265 (comment) @emilio suggested that we should make the changes to add a style="display: block; content-visibility: hidden;" when a <details> closes and remove it when opening be part of the UA style sheet rather than style attribute manipulation written into the algorithm.

This seems likely to be a good idea. We should try to both implement and specify it.