(') and double straight quote
Typography purists would point out that proper foot and inch marks have a slight northeast-to-southwest slope to them. HTML purists would further point out that these characters have their own escape codes (
″). True, and you are welcome to seek them out.
What you will find is that even though these characters are available in system fonts, they are less common in professional fonts (found in font recommendations). So as a rule, straight quotes are the most reliable tools for the job.
You can italicize the straight quotes if you want an even better approximation of sloped foot and inch marks.
Tread carefully if foot and inch marks appear within quoted material.
Why would professional fonts omit these characters? Foot and inch marks have never had a direct key shortcut on either Mac or Windows, nor have they been part of any standard character-set specification, so they have gone obscure in the digital era. But they are starting to reappear.
If you’re a purist whose chosen font is missing true foot and inch marks, there’s nothing wrong with using the marks from a different font. Match the height and weight as best you can. No one will be the wiser.
A nonbreaking space in the middle of a foot-and-inch measurement will prevent an awkward linebreak like 6'
10", and ensure that you get 6' 10".