Hi Scott,
I am afraid that for InterwebPPC and Netflix, it is "slightly" more than just having HTML5 as it is a markup with some additional protocols supported. In the case of Netflix, it requires the secure content streaming thingamagee and it would require some determined development to try and get something (if ever possible). Think about the TLS blogposts and the challenges to deal with the crypto and multiply by 10 just for the fun of making sure nobody can watch "game of naked dragons" for free.
On the Whatsapp site, as long as there is no glitch like i.e. on LinkedIn, you should be able to work around such browser checks by changing the browser identifier (and I think you can save the pref per site).
And finally, for OS9, classilla is still head and shoulders above everything else but the crypto may be lacking for some sites so you may need to resort to some crypto ancienne tricks to proxy your traffic to retro-freedom