After all, a modern cell phone could be producing 4k 30fps video, and high performance barcode decoders will detect linear barcodes with lines a single pixel wide, even in the presence of moderate noise/blur/overexposure (e.g. the barcode on a shiny can of coke)

Of course, I've got no idea if browsers will provide high performance barcode reading, or something simpler like zxing.

And I've got to say, it's not obvious to me why browsers would implement a barcode scanning API in preference to a more general accelerated image processing API. Plenty of other applications want to do real time video processing in the browser - such as video call background removal.