How often is the default font size in the browser not 16px?
After writing a great big answer to the question Is it bad to work with pixels in CSS? in which I concluded that there's normally nothing wrong with it provided you understand the limitations, I started to wonder about how many people have the default font size set to something other than 16px.
I imagine that there would be some users who change it due to either poor eyesight or high pixel density monitors (like those with 1920*1080 screens on 15" laptops) (or maybe just because they like larger text). There also may be some devices (handheld devices?) that have the default font size set to something else, but this is purely conjecture on my part.
My question is: does anybody know any statistics on what percentage of users have the default browser font size set to something other than 16px?
This avoids answering your question, but IMHO such statistics shouldn't matter even if it were near zero percent.
Even if 16px were the de-facto standard, you should start from the position that this is something that the designer doesn't ultimately control, and it should be left up to the user to choose their font size. Your decision process should lead to designs that are flexible based on the user's needs.