It has the same effect as shouting. Don't overuse italics Text in italic is hard to read on a screen. The resolution of a screen just isn't capable to present italics without distorting them slightly. This is even more noticeable if you use a small font. So don't use italics for larger portions of your text.
Small letters are hard to read and that's even worse on a computer screen.
Unless you run an online font-archive. Do use punctuation Present information surveyable. Present it in small chunks. Use headings to separate them. Use lists to avoid long textual summing ups.