The worst is not Outlook, you can get it to do what you want most of the time... the worst are the myriad of webmail clients. They do not use iframes (which would be the best way, given you can limit what an iframe can load - scripts, images, other external resources - pretty good), but they filter and rewrite the markup each in a different way so they can inject it more or less safely in the primary markup...
And next upon the list are the Android mail clients, they're a mess, too... and there is no practical way to do automated tests or even comparisons, you gotta test 'em all by hand.
I assure you, I’ve had a lot more trouble on the MSO renderer than others. Sure, webmail clients are commonly fairly dodgy, and Android clients are pretty terrible, but the MSO renderer is the biggest one that seems to actively go out of its way to break your stuff. It’s the one that simply blows up in weird and wonderful ways rather than merely failing to use a declaration.
At FastMail, we’re looking forward to using JMAP in production, because then the defanging of HTML emails will be done on the client side rather than in a rather ancient hunk of Perl that works tolerably well but we’ve mostly avoided touching because JMAP is coming (still, it’s already fairly good as webmail clients go).
The worst is not Outlook, you can get it to do what you want most of the time... the worst are the myriad of webmail clients. They do not use iframes (which would be the best way, given you can limit what an iframe can load - scripts, images, other external resources - pretty good), but they filter and rewrite the markup each in a different way so they can inject it more or less safely in the primary markup...
And next upon the list are the Android mail clients, they're a mess, too... and there is no practical way to do automated tests or even comparisons, you gotta test 'em all by hand.