I doubt whether it has anything to do with a codec. KM 75 handles all the HTML5 elements just fine, including the audio and video players.
The lesson buttons don't even bring up the page they're supposed to open, although the appropriate URLs do appear in the address bar. Another development, however, is this: I tried removing all the blocked URLs from my Javascript permissions list (I could have simply chosen to allow them, but I was a bit too hasty). Oddly enough, this finally made duolingo.com crash the browser shortly after loading the page. I don't know which of those permissions made the difference, however, and the list was very long, but I've since restored them from my backed up profile. At the same time I removed Duolingo from my bookmarks.
Anyway, I tend to think that the fault is with the site, not the browser -- sloppy script design, in other words -- and as has been noted earlier in this thread, KM is not very fault tolerant. When I used IE a long time ago it was way
too tolerant; I could forget to close important tags in my own HTML pages and IE let me get away with it, because the pages worked anyway. People viewing them on other browsers would have complained bitterly.
I'm more computer savvy? No way; look at all the help I needed with those macros!
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Gerry