What's the difference between and , and ?

Answer 1

Here's a summary of definitions together with suggested usage:

...a span of text to which attention is being drawn for utilitarian purposes without conveying any extra importance and with no implication of an alternate voice or mood, such as key words in a document abstract, product names in a review, actionable words in interactive text-driven software, or an article lede.

...now represents importance rather than strong emphasis.

...a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, a thought, or a ship name in Western texts.

...indicates emphasis.

(These are all direct quotes from W3C sources, with my emphasis added. See: https://rawgithub.com/whatwg/html-differences/master/Overview.html#changed-elements and http://www.w3.org/TR/html401/struct/text.html#h-9.2.1 for the originals)