That is, if their browser supports style sheets properly. Type attributes like bold, italic, underlined and so on have various functions. These functions include emphasizing certain words or phrases, or marking them as special for some reason. Which attribute should be used for which function depends on what language you're working in, and what style guide you choose to follow. Typography and punctuation can vary quite a bit between British and American English, and even if you stick to one side of the pond, the experts don't all agree on what's correct and what's not.
Bold text is used for headings.
Never use Capitalized Words, or worse yet, ALL CAPS, for emphasis. Italics are traditionally used for emphasis. As for what's foreign and what's not, I won't even touch that one. Italics are also sometimes used for extended quotations, or to identify a word that the writer figures the reader isn't familiar with. In the latter case, the word is defined soon after being used for the first time, and only the first instance is italicized.