What does the i tag do in HTML?
The i element is used to mark words in the surrounding text, highlighting text in italics. The i tag is essentially a typographic element and is embedded (not blocky). This means that it should be used to mark up multiple words or phrases.
The i element is used to mark words in the surrounding text, <i>highlighting text</i> in italics.

i or em?
The i element highlights text in italics. The em element points to text that is italicized. So what’s the difference? The em element emphasizes the text, and the i element indicates that the text should just be italicized. So, when should you use one and when should you use the other? There are rules for highlighting text: if you want to emphasize part of the text, you should use the em element; if italics depends only on style (Latin typographical abbreviation), you should use the i element.
The element i <i>highlights text</i> italics. <p>The element em <em>points to text</em>, that is italicized.</p>

And don’t forget dfn
The dfn definition element, which is used to mark up a word or phrase in text, also italicizes text by default. However, the rules for using this element instead of i are much simpler. It should only be used when marking up a definable word.
The dfn definition element, which is used to mark up <dfn>word or phrase</dfn> in the text.

Long-form italics
Sometimes it is desirable to italicize several lines of text. This can be a brief introduction to the content, a note to the reader, a “fine print” of one kind or another. The i element should be avoided here as well. If you want to italicize an entire paragraph, assign it a specific class or id and use CSS.
<p class="text">Sometimes it is desirable to italicize several lines of text. This can be a brief introduction to the content, a note to the reader, a "fine print" of one kind or another.</p>
<p class="text">1 You should also avoid the element i here. If you want to italicize an entire paragraph, assign it a specific class or id and use CSS.</p>
p {
font-style: italic;
}
