I'm running into this issue right now with the locked UA. Here are the challenges: 1. ios 11.3 safari bug broke <img src=mp4> despite advertising `accept: video/*` in the request. (see bug 185029). I now need to use UA detection to patch versions of a response based on existing bugs. In short, we as an industry, depend on UA parsing to work around bugs. This happened last year when the jp2k support increased to 10bit depth and the decoder was inflating the 8bit colour pallet to 10bit. We needed to use UA parsing to determine if it was safe to send jp2k images to a device. I have a laundry list of other examples where the server might actually know more than the client to prevent a) a broken user experience and b) prevent cellular data waste.