You can still download the
Java 1.1 SDK from sun... anything developed with this is going to be 1.1 compatible.
Actually, I would develop with 1.3 ( or the new 1.4 ) and just
test it with a browser without the Java plugin installed to make sure that it works... the majority of things to steer clear of if you want to be 1.1 compatible are Swing, Java2D, and the Collections classes ( stick to using Hashtable and Vector only ). There are a few other things, such as new methods introduced after 1.1, but these are usually pretty easy to fix, and you can check in the API to see when methods and classes were introduced.
-Nate