Bolded or italicized text are used to emphasize or draw the eye to important concepts, ideas or information in the content of a webpage. Webmasters can achieve the look through the use of the use of four HTML tags: <i>, <em>, <b> and <strong>. The <i> and <em> tags display text in italics, and the <b> and <strong> tags display text in bold. Each of these tags provide a little bit of value to search engines for those keywords that are emphasized while serving a purpose, visually, in the content of a webpage.
The key difference between the <i> and <em> tags lie in presentation versus structure. To the average user, the tags could be used interchangeably as both display text on a screen in italics. A screen reader approaches each tag differently, however. When reading content, a screen reader will overlook the <i> tag as a visual element. When reading the <em> tag a screen reader will say the text with emphasis.
Like with the <i> and <em> tags, both the <b> and <strong> appear similar when displayed on a screen. When read by a screen reader, the <b> tag will be overlooked, while the <strong> tag will be spoken to a user with strong emphasis. The <b> tag is presentational; the <strong> tag is structural.
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The alt attribute is used to note alternative text (alt text) information to be rendered when the elements to which the alt text has been applied are not available to be rendered (e.g., missing image file, missing video file). The alt text is also utilized by screen readers (generally used by the visually impaired).
A visually impaired reader will hear the alt text in spoken language so the description should be used to impart the exact information that the image is meant to do. For example, if the image in question is of a flag, and the flag is being used not necessarily to refer to a country, the alt text can read something along the lines of “A flag blowing in the wind.” However, if the image is meant to refer to a specific country flag, then the alt text could read “The U.S. Flag.” So the alt text is based on the intention by which the image was used.
For search engine optimization purposes, the alt text can be used to not only describe the intention of the image, but to also incorporate keywords which are relevant into the alternative text. It is important to remember that the alt tag is not to be used to simply include a list of keywords or a sentence stuffed with keywords.