Only a small variation in the text is required to establish visual contrast -- There is no need to overkill by using multiple styles at the same time.

Italics: Use italics for quotations, foreign words or for inconspicuous stressing. Avoid large blocks of italics because they are less readable than normal text on-screen.

Bold: The human eye is very receptive to differences in brightness within a text body. Boldface text stands out in contrast to normal text and is very readable on-screen. For easy visual scanning, use the boldface to highlight keywords which are important to the subject of the text. Avoid large blocks of boldface text because they will lose their effectiveness.

Underline: Although used as a means of emphasis in print and written material, underline has been widely used to indicate hyperlinks in web pages. Avoid using underline for non-linked text to avoid confusion.

Color: Colored text is increasingly being used as an alternative to bold and hyperlinks. The convention is to use color in headings and avoid it, as with underline, within regular text. This also applies to text highlighted by a colored background, which should be used sparingly.

Capital: We have learned to recognize words both by their letter-grouping and by their lower-case shapes. Although attracting attention with their rectangular blocks, a block of text in capital letters can be difficult to read because we are not accustomed to only identifying words with their letter-grouping. Confine capitals to headings, warnings and titles. Avoid large blocks of text in upper-case because it significantly slows reading.

Spacing: Medieval German scribes inserted spaces between letters to highlight a word in their very dark blackletter typeface, achieving the effect of a reverse-boldface. We can still apply this technique by using white (or negative) spaces to highlight a paragraph or words in the midst of dense text.