Allowed characters in CSS 'content' property?

[watchman]

I've read that we must use Unicode values inside the \[code\]content\[/code\] CSS property i.e. \[code\]\\[/code\] followed by the special character's hexadecimal number.But what characters, other than alphanumerics, are actually allowed to be placed as is in the value of \[code\]content\[/code\] property? (Google has no clue, hence the question.)