This may be common knowledge but it confused the heckins out of me for a good 20 minutes! The Web Programmer appears to cache the .bin file, so if you change the contents of the firmware binary (i.e. recompile) and then try to upload it a second time, it will upload the original version again (even if you browse for the file again). You need to refresh the browser, which clears the file, and seems to clear the cache.
I’ve generally used the STM programmer, so haven’t had much experience of the Web Programmer. Just thought I’d put this up here in case it helps anyone else. Would be nice if it could be fixed at some point, but I suspect it’s low urgency.