It looks like more than that alone is happening. I assume the "3D"s are hex for the equal sign, which would be redundant and AFAIK likely cause breakage. And the single quotation marks around the font family are entities instead of single quotes. Then there is an empty namespaced p element which I’m guessing is for the default spacing instead of using CSS to create spacing. etc.
Maybe the Outlook email client knows how to deal with that mess, but I don’t think it should be expected that anything else would be able to.