There are a few major players in this realm including jQuery, mootools, prototype, and a few others. These libraries spend a considerable amount of processing time just checking the browser type and a percentage of their size is useless to each browser. Also, many web developers consider their sheer size as a large minus when choosing libraries.
The necessary steps are detailed below:
- Web developers would need to use the version of the library in the filename, such as jquery.1.4.4.js or prototype.1.6.js, in a very parse-able way.
- Browsers or extension developers would create a plugin/extension/feature that implements the localized library extension
A possible way to go about #2 would be to use pynarcissus (http://code.google.com/p/pynarcissus/) in some way to find anything that may be a browser check and create browser-specific versions through that. I would work on this, but I don't have time so it's just an idea for now.