Why doesn't the CSS calc() function work for me?

Question

I learned about the CSS function calc() and its awesome. I tried to use it like:

#abc{width:calc(100%-20px)}
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Answer 1

All the modern browsers except Android's native browser support calc() which makes it easier to adopt. But do note that it can have a big impact on the layout and the consequent breaking of your design on unsupported browsers.

You should use it with a fallback declaration, so it doesn't break browsers which don't support it.

width: 500px; /** older browsers **/ 

width: -webkit-calc(50% - 20px); /** Safari 6, Chrome 19-25 **/

width: -moz-calc(50% - 20px); /** FF 4-15  **/

width: calc(50% - 20px); /** FF 16+, IE 9+, Opera 15, Chrome 26+, Safari 7 and future other browsers **/