That's actually not a bad idea! The HEIF examples on the Nokia HEIF site do something similar; they implement a HEVC decoder (and do the HEIF container process

That's actually not a bad idea! The HEIF examples on the Nokia HEIF site do something similar; they implement a HEVC decoder (and do the HEIF container processing) all in JavaScript using http://www.libde265.org http://www.libde265.org. The images you see are decoded into a HTML5 canvas.