Currently the preview pane is completely un-styled, and you are expected to add your own.

I think for starters and minimal use-cases, it would be useful to include some default styling. For example setting the fonts, paddings and margins to that of the rest of the interface, and making images responsive.

Not everyone is interested in providing custom styling but the un-styled defaults are unusable. I think this is a bad first impression for new-comers too.