Bold, Italics, and All Caps
Bold, italics, and all caps should be used sparingly. They’re intended for emphasis and if everything (i.e.: a whole paragraph) is all caps/bold/italic, nothing is emphasized.
Italicization is used for gentle emphasis, bold is used for heavier emphasis. All caps are another alternative means of emphasis. None of these should be used on more than a few words together. Think more in terms of emphasizing keywords. Remember, in web pages, underlining should only be used for links.
For more information, read the following article at http://practicaltypography.com/bold-or-italic.html.