Have you specified Kind of Project: HTML?
Yes, thank you, I found it in the end. But why I should to mark it, if regardless of this setting, I can mark downloadable Windows and macOS as well but not vice versa? And my game having both – downloadable and HTML... Seems like legacy part that allows to select even Flash and old Unity that is not supported by modern browsers. I just want to mark WebGL playable regardless of this setting, as I'm doing it for downloadable types.