There are reasons this is so hard - some involving performance, but most importantly security. I think you have one of two situations, both of which are bad:
- you’ve written a backend that produces HTML
- somebody else has written a backend that produces HTML and you have to blindly trust that there isn’t anything dangerous in it
So either way, I would urge you to reconsider HTML as a backend transfer format for Ionic apps. Use JSON instead and something like this skeleton idiom to render it.