Is -Moz-Calc a CSS preprocessor?

That's actually LESS, a CSS preprocessor. The ~"" syntax creates a literal value which doesn't process its contents as LESS. Otherwise, it would try to compile -moz-calc () as a LESS function. Thanks for contributing an answer to Stack Overflow!

What is -Moz-calc() function?

display:block; width: -moz-calc(100% - 2 * 3px); One particularly powerful feature of the calc() function that you can combine different units in the same computation: The current implementation supports the +, -, *, /, mod, min, and max operators.

How does calc() work?

The calc() function takes a single expression as its parameter, and the expression's result is used as the value for a CSS property. In this expression, the operands can be combined using the operators listed below. When the expression contains multiple operands, calc() uses the standard operator precedence rules: Adds the specified operands.