Try as hard as you can to avoid using bold and italics. Bold text would make readability extremely difficult in most RTL languages (especially Arabic), and italics is simply never used. As well, capital letters are almost always disregarded in RTL languages.
If you need to highlight a certain part of a text in Arabic, overline it instead of underlining, interspacing or italicizing. You can use the overline text-decoration CSS property.
Make sure that all text is aligned to the right, and that font and font size are adjusted properly (better yet, customize them) because Latin fonts tend to affect Arabic readability rather poorly.
Also, remember that words in RTL languages are often much shorter than words in English. So, adjust to keep a balance in how text is displayed on the page.