Yes, HTML5 will be a pain to develop for if you are still using a text editor. Not so much if we use full featured mature web development environments.
I'm not sure if you are arguing for or against html. Sure, a closed proprietary authoring tool (Flash) that runs in it's own player will be easier to author, but you must realize that you are not making a web site, your making a Flash site which is not part of the web/internet/html standard, it is only an extra - that should never be required of a user to have on most normal web sites. Use it all you want, so many people do, but Flash developers are not creating web content; only Flash content. I don't know of a single web browser that can render Flash content without additional non-web browser software.
It's a chicken and the egg sort of problem which many of us have seen from the beginning, recognized, steered clear of, or at least attempted to avoid. I have needed to tweak content so it's renderable in multiple browsers to make a client or two happy just like every one else, but I felt really dirty afterwords every single time.
This is a problem we create for ourselves by ourselves. Way too many of us have done this (some more than others) writing non standard html for specific browser features and not writing code to the spec; which is in fact the solution. If the code, written to spec, does not work in Browser X. Stand strong argue against it to your clients; we must not capitulate. If it's the fault of Browser X, then F**K Browser X and make them fix their own problem themselves and let the site be broken (and if you can, add a comment on that page to shame that Browser at the same time so your users know the bug is their Browser and not your site).
None of us would put up with a bad software compiler that requires us to write non standard language code in order for our compiler to compile it correctly. We would find an updated one without the problem or revert to a older version without the problem. Yes, it's hard to argue to clients, as each user in the case of the web chooses their own compiler (Web Browser), but it needs to be done. Only write to the html spec, that's how we'll get ourselves out of the pickle.
