I thought this was objective enough for SO, but it's too open-ended. Hopefully it won't be too objective here, because if so, I'm out of SX sites. Here it goes:
- When a browser handles a standard js in a non-standard or atypical way
So with the above in mind, if a browser offered a unique JS method that was not available via standard JS, that wouldn't really fit the list (unless the method later became standard and the original browser neglected to update to the standard).
I know there are lots of sources for these discrepancies, but I haven't found a solid "quirksmode" like list that I can turn to, so I'm hoping this question might become such a list. If someone knows of a really good source already, by all means tell me and consider the question closed.
The other goal I have in mind is a way to measure various js frameworks; is the discrepancy addressed and how well is actually handled.
The power of a good js framework like jquery is that it provides a means of writing code that all major browsers will treat the same, even if they don't use the same prototypes or differ in interpretation. The most obvious examples of this (that I know of) that jquery addresses well are:
Ajax Request and Response Handling
As far as I know, jquery does not have any methods that resolve/unify XPath requests (there's probably a plugin, but that's not the point).
It's these types of differences I have in mind. Differences so major that without a framework you end up writing two or more sets of the same basic code or differences so minor that you could lose a day debugging or refactoring because you have no way of knowing (without a handy list) that browser X treats a standard DOM property differently inside a for loop setup. The gremlins that on a good debugging day you finally catch, but no one else would even think to Google for when it comes their way.
I also don't think there is as much need for listing a quirk in IE3, Safari 1.1, IE for Mac or Camino or elinks. If you know some humdingers and want to provide it, it can't hurt, but don't mistake my use of "all" to mean "every last one, or the kid gets it!" I mean "all the big differences that a good framework should address" plus "all the differences a web developer should consider first when their code works in one browser but not another."