Quotation marks are for quotations
As with underlining, quotation marks are not to be used for emphasis; they are used when quoting someone, using the q or blockquote tags or — sparingly — to show irony.
(Irony- or scare-quotes won’t be marked-up in HTML as quotes. Italics are preferred over quotation marks in non-quotation situations.)
Smart quotes
Word processors’ smart quotes settings have saved us from the embarrassment of inappropriately using non-curly single- (‘ & ’) and double-quotation marks (“ & ”) and apostrophes. Which is great. The downside is that it can be hard to get an inch or foot mark when you need one.
Foot & inch marks
In HTML, we can use either straight quotes (" for inches, ' feet) or their slightly-slanted equivalents, called primes (′ & ″), which are technically more correct.