Why does bold and italic text not work?

Try deactivating all plugins to explore whether any could be responsible.

That is NOT the original Twenty Eleven theme!

It isn’t. I just deactivated all plugins and the problem still exists.

An edited version of the theme Twenty Eleven

Can you replicate the problem using the default Twenty Thirteen theme?

How would you suggest doing so?

You go to Appearance -> Themes and you activate the Twenty Thirteen theme.