I did some proof-of-concepts ages ago (October 2010) exploring different ways of embedding a PNG alpha channel in JPEG application specific fields to have 100%

I did some proof-of-concepts ages ago (October 2010) exploring different ways of embedding a PNG alpha channel in JPEG application specific fields to have 100% JPEG compliant files which web browsers could display with transparency. It worked just fine. If you want transparent JPEGs on your web page that works. Back then the number of GETs was really important, so stuffing the mask into the JPEG made sense. Now with our HTTP/3 and QUIC world that isn't such a big deal.You might be better off just using a CSS mask image.