There's plenty of resources out there for 'Browser Stats'. it would really depend on the projects target audience. I have not personally seen any numbers on DPI

There's plenty of resources out there for 'Browser Stats'. it would really depend on the projects target audience. I have not personally seen any numbers on DPI%, but I wouldn't be surprised if they exist. You can achieve some flexibility in layout for device, resolution and pixel ratio constraints using CSS media queries, it just depends how granular you want to go and how much time it takes to implement all the use cases from a business perspective (from experience)