I was going through last year's Google I/O conference videos and noticed a few GWT-related ones. One in particular seemed interesting: Faster-than-Possible Code: Deferred Binding with GWT by Bruce Johnson.
Our system would generate several pages from a common definition written in CL-WHO and Parenscript: we had both browser-specific code and automatic metaprogramming-based profiling instrumentation, so in all the code generation outputs would be a cartesian product of (Internet Explorer, Firefox) x (regular, profiled). The resulting combinations were served under different URLs that also incorporated a version number (via a mechanism that linked source files to generated resources and tracked the file modification date). This approach generated perfectly cacheable HTTP resources and ensured trouble-free application upgrades.
Something interesting that I noticed when developing the feature-specific code generation mechanism was its resemblance to context-oriented programming: each feature acts as a layer which affects the way that the compiler produces code.
Besides browser-specific code generation, in GWT the technique is used for generating locale-specific resources, and by virtue of its implementation, to provide an extremely obtuse way to emulate Java's broken metaprogramming facilities (see this tutorial for example).
In the intervening time I have learned a little more about programming
for the web, and have come to the inevitable conclusion (if I had
I could have avoided making this mistake) that using the approach to
generate browser-specific code is fundamentally flawed and will almost
certainly ensure that your code will break on browsers that you have
not developed for. Working to add support for new browsers to this
scheme will not only waste an incredible amount of time that you would
not have had to spend at all otherwise, but will actually make your
code more brittle and harder to maintain.
The only viable approach to dealing with differences in browser capabilities is feature detection. I can only say that I am glad that Parenscript is not pushing a "framework" on anybody, so my ignorance only impacted one project. Generating feature-specific code is not in itself a bad idea - GWT uses it to also generate locale-specific resources, with the corresponding bandwidth savings and cacheability advantages. I can only hope for the sake of their users that the GWT developers repent and change their stance on generating browser-specific code.