To my knowledge there isn't a way. I know that only certain html is allowed and that is one reason some stuff get's re-written. There was a html whitelist document but I'm having trouble locating it.
I know it can be frustrating but I also get why Canvas wants to limit some of what is allowed and "correct" some html to be formatted how it wants. I have a case open right now about a course where the link validator tool won't complete and apparently it's due to some bad html on one of the pages. Point being that bad html can break functions so trying to keep it "clean" as much as possible probably helps some with that even though it can be annoying for power users.
Rick
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