The absolute bare minimum is to show text as it was written. Most of the apps don't even deal with formatting post correctly, so I am happy to stick with a browser.
If your app of choice does not display the following correctly, let the devs know via a bug report
Bold = Bold v. normal.
Italic = Italic v. normal.
Superscript = SuperScript v. normal.
Subscript = Subscript v. normal.
Strike-through = Strike-through v. normal.