Pushing JS, and HTML to the limit always feels like the biggest con of building javascript UI apps. The farther you move away from browser defaults the more hairy thing seem to get.
No doubt we are moving to a more dynamic client side, but I haven't seen the way forward yet.
Do you feel that way with modern browsers too? We have not been paying much attention to IE compatibility but things have been working good in Chrome/FF/Safari for most part. Also, frameworks like Backbone help you write code thats easier to debug and more or less runs out of the box on most modern browsers
Thing are getting much better, all the time, but simple things like progressively enhancing select box's just feel way harder then it should be. Select boxes are tricky in any browser by the way because they are natively rendered so differently in many different browsers.
Backbone, and others, are awesome but still don't seem like the whole solution.
Even then if you the site you are building gets a little more popular, you will have to support more platforms, like older versions of IE, and then were back at the start.
The problems we got with older browser versions(especially IE) is mainly with CSS. Not JS or HTML.
Rendering on the server would have caused the same issues.
I think in recent versions this is true (the problems being mainly CSS). 4-5 years ago I remember doing JS DOM manipulation and at one point IE taking 100x longer to do something than Firefox. It was so bad that FF was instant and IE was unusable.
No doubt we are moving to a more dynamic client side, but I haven't seen the way forward yet.