A lot of markdown note apps including FSNotes (https://fsnot.es) support the TextBundle (http://textbundle.org) format so you can embed images in the same document.
HTML is nice for more advanced documents, but I prefer note taking in markdown as it's a simpler syntax, even when viewed in raw text.
I think HTML is perfect for this app because it's more for academic use and less personal use like grocery/todo lists, reminders, etc.
Having a synced mobile version is crucial as you want to access your notes on the go right? (at the grocery store, etc.)
I considered textbundle, but went for HTML for one reason: data accessibility. Imagine in 20 years from now you'd want to access your data and do not have the app around for some reason. You would need to find another app that supports the .textbundle format. Or you would need to manually rename all .textbundle notes to .zip to extract them. Even if a normal user would know about this, it would still be a pain. With HTML you can drag a note into a browser and see the contents. Links to other notes will work as well. Images will show right away, too. Pros and cons for each format...
Well that's one reason to not use a subscription based app. In the situation of a one time paid app a normal user would continue to use the same app, a power user would know it's a simple zip file. If you use a subscription based app you may not renew and you may be shit out of luck. A normal user may not even know what an HTML file is, much less how to edit it with a different program.
If you don't renew you can still use the app and open and edit all your existing notes. Good point, I need to make this more clear on the site. HTML vs. ZIP vs. power user: I'm going for the most common denominator, the average user with a browser :)
This looks good if you are looking for something a bit more for school or something.