Thanks a lot for the answers and the many different options!
I think Kompozer (thanks, MarkMLl!) is really the closest to what I expect.
Bootstrap Studio (thanks, xinyiman!) is interesting, but it is also paid and, of course, directly dependent on Bootstrap, which is not ideal.
Bluefish (thanks, munair!) looks like a good alternative to NetBeans, but both of these apparently do not provide the direct visualization and manipulation of html elements at design time that I am looking for.
As suggested by john horst (thank you!), there are many more hits with the WYSIWG keyword. I will see if among these there is an up-to-date alternative to Kompozer, but this might take a while. I might just stick with Kompozer for my purposes.