There's a sheet method declared but not implemented in the Objective-C DOM wrappers.
Created attachment 3247 [details] patch to implement the method
Comment on attachment 3247 [details] patch to implement the method Patch looks good, but needs some sort of testing. r=me if you find a way to test it.
Comment on attachment 3247 [details] patch to implement the method Looks great. (this code actually benifits from the odd behavior of the last patch, in that it will "correctly" return a DOMCSSStyleSheet or DOMStyleSheet depending on the impl.) r=me
As ggaren correctly points out, all of these really should have layout tests. For better or worse I ignored that fact, realizing that we don't currently (AFAIK) have any way of testing the Obj-C DOM API. I talked with andersca a bit on IRC, he said he started the py-objc Obj-C DOM testing system, but hasn't finished yet.
Note that the "odd behavior" that Eric mentions here is the design of the DOM wrappers. The class of the Objective-C DOM wrapper mirrors the class of the actual DOM object it's wrapping. That way you can use standard Objective-C techniques like "isKindOfClass:" and downcasting on the wrappers.