For development purposes, I'd like to get a rough estimate of the distribution of browser usage share, but everything I've been able to find just states overall market share. Is it possible to get browser share by OS?
I would do this on a case by case basis per client. If you are developing a new site for them I would look at the analytics data of their existing site to determine the browser by OS of their existing target audience and then develop the new site accordingly.
I don't know of any existing data sources that show this in general although I am sure they exist somewhere.
Go to NetMarketShare and filter by Operating System at the top. Note that you need to subscribe to paid "Geolocation Upgrade".