[html] `—` or `—` is there any difference in HTML output?

They are exactly the same character. See: http://en.wikipedia.org/wiki/Dash

Barring browser bugs they will display the same in all cases, so the only difference would be concerning code readability, which would point to —.

Or, if you are using UTF-8 as a charset in your HTML document, you could enter the character directly. That would also display exactly the same.