If you're that worried "<p> hold shift to select more than one item.</p>" isn't a whole lot extra to add and is something most people can handle.
Never done tech support I take it!
It makes me think when I'm designing things. If I have problems with stuff like that, how can I make it better.
Then there's the way styles are so different, especially on the web. Horribly user-hostile. How does this site mark something as important, assuming it bothers in the first place? Who knows. And if I'm only planning to be on it for 30s, I'm not going to learn that.
It's a standard UI control and it is used pretty often.
Or you could just use a better control.
Many users don’t even know you can use a key shortcut to copy/paste.
"‡ On a phone or tablet or whatever, ignore us, we're just confusing you."