tag i vs. tag em, ¿cuál es la diferencia?

<i> vs. <em>

It is often confusing to new developers why there are so many elements to express emphasis on some text. <i> and <em> are perhaps one of the most common. Why use <em></em> vs <i></i>? They produce exactly the same result, right?

Not exactly. The visual result is, by default, the same - both tags render its content in italics. But the semantic meaning is different. The <em> tag represents stress emphasis of its contents, while the <i> tag represents text that is set off from the normal prose, such as the name of a movie or book, a foreign word, or when the text refers to the definition of a word instead of representing its semantic meaning.

An example for <em> could be: "Just do it already!", or: "We had to do something about it". A person or software reading the text would pronounce the words in italics with an emphasis.

An example for <i> could be: "The Queen Mary sailed last night". Here, there is no added emphasis or importance on the word "Queen Mary". It is merely indicated that the object in question is not a queen named Mary, but a ship named Queen Mary . Another example for <i> could be: "The word the is an article".