But, like, of course they are. This is what HTML was built for. The other major standard would probably be RTF, but it's a bit less structured, and so less close to Markdown. HTML is the better pick.
If you subsequently want to style that HTML, so that every second-level heading uses a specific font, and every third-level heading uses some other font, and so on, CSS is the best way to do that.
So, yes, we're saying the same thing, but to me it's a bit like saying "If you want to find the answer to 2 + 2, addition is the only viable option." Well, yes!
I think the reason this feels kind of wrong is because that same HTML and CSS renderer you're using for Markdown also comes with an entire 3D graphics pipeline and audio synthesizer. Obviously, we should be able to answer 2 + 2 without opening Mathematica.
I guess the important technical question is whether simply creating a WKWebView also loads in all that other stuff. I would hope and expect the OS is smarter than that, and you can call WebKit for simple HTML without everything else coming along.
