Tilde in CSS attribute value

What is it?

A syntax error.

What does it do?

Breaks your stylesheet.

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As mentioned in the comments on your question, it may be LESS instead of CSS. In LESS, a tilde before a string literal outputs the string as-is, because it may be a syntax error in pure LESS. \9 is an IE hack. See Escaping.

It may be a LESS Syntax. But tilde character is used in CSS3 as well. E-g the below Syntax:

p ~ ul { background:#ff0; }

will target ul elements that are preceded by a p element with the same parent: