Ever since Google decided to penalize link selling, webmasters use nofollow even when they shouldn’t. That has been at least my experience. I guess a lot of people think it’s better to be safe than sorry.
I have spotted a question on Twitter whether nofollow would work in single quotes too.
— Gary "鯨理" Illyes (@methode) October 9, 2015
Gary Illyes responded that it doesn’t really matter. Single and double quotes are both fine.
What is Valid HTML?
Now if you would ask me, I would use double quotes. I think that’s what valid HTML is. There are a lot of theories what does Google consider quality, and vali HTML markup is suspected to be one of the indicators.
Quality signals are used to rank websites, and in updates like Panda and maybe Penguin (although it’s considered an offsite algo).