Fonts
Only use the fonts that the user has specified in their theme, and in sizes relative to the default size specified in their theme. This will ensure maximum legibility and accessibility for all users.
Do not mix more than two or three font sizes and styles (underlined, bold, italicized) in one window, as this will look unprofessional and distract the user from the information being conveyed.
Provide alternatives to WYSIWYG where applicable. Some users may need to print text in a small font but edit in a larger screen font, for example. Possible alternatives include displaying all text in the same font and size (both of which are chosen by the user); a "wrap-to-window" option that allows you to read all the text in a window without scrolling horizontally; a single column view that shows the window's contents in a single column even if they will be printed in multiple columns; and a text-only view, where graphics are shown as placeholders or text descriptions.