To bold text, use the <strong> or <b> tags:
<strong>Bold Text Here</strong>
or
<b>Bold Text Here</b>
What’s the difference? Semantics. <strong> is used to indicate that the text is fundamentally or semantically important to the surrounding text, while <b> indicates no such importance and simply represents text that should be bolded.
If you were to use <b> a text-to-speech program would not say the word(s) any differently than any of the other words around it - you are simply drawing attention to them without adding any additional importance. By using <strong>, though, the same program would want to speak those word(s) with a different tone of voice to convey that the text is important in some way.
To italicize text, use the <em> or <i> tags:
<em>Italicized Text Here</em>
or
<i>Italicized Text Here</i>
What’s the difference? Semantics. <em> is used to indicate that the text should have extra emphasis that should be stressed, while <i> simply represents text which should be set off from the normal text around it.
For example, if you wanted to stress the action inside a sentence, one might do so by emphasizing it in italics via <em>: “Would you just submit the edit already?”
But if you were identifying a book or newspaper that you would normally italicize stylistically, you would simply use <i>: “I was forced to read Romeo and Juliet in high school.
While the <u> element itself was deprecated in HTMl 4, it was reintroduced with alternate semantic meaning in HTML 5 - to represent an unarticulated, non-textual annotation. You might use such a rendering to indicate misspelled text on the page, or for a Chinese proper name mark.
<p>This paragraph contains some <u>mispelled</u> text.</p>