Can you elaborate more? Simple example, rendered with pythonista serving versus another server?

The built in webview uses the older UIWebview. A more modern equivalent is the WKWebView -- see the @mikael
Repo here

Of course, if you are seeing the same issues with a proper desktop browser, then the problem is more likely that the css is not actually being served... Developer mode in chrome etc would let you figure out if the css is actually getting served properly. A common problem in pythonista is dealing with paths correctly.