Ref.   
https://play.sonos.com/en-us/browse/services/130823 pops up https://www.90s90s.de/

https://play.sonos.com/en-us/browse/services/82183 pops up https://www.80s80s.de/

https://play.sonos.com/en-us/browse/services/61191 pops up https://www.audible.com/

https://play.sonos.com/en-us/browse/services/46343 pops up https://www.mixcloud.com/

where others like Apple Music, MyTunerRadio, Sonos Radio (of course) or BBC Sounds are working fine within the intended one tab confinement.

When will these (and other similar examples) work within the new web app instead of having to go to their homepage to airplay to Sonos? ...or cast to poorer-sounding alternatives like Google Home?

If Mac and Windows controlling are on the way out (maintenance mode) then the web app needs to shine and there is plenty of space for improvement ...including different background colors (for once). 

...or is this geek doing something wrong? (latest Chrome on latest macOS on relatively new macbook air on decent fiber-optic broadband) please tell me if so.