When nesting occurs in the CSS calc function, WebKit fails to calculate the result.
Example (works in Chrome, empty in Safari): http://jsbin.com/tonova/10/edit?html,css,output
This is quite common when using CSS variables, e.g. http://jsbin.com/tonova/11/edit?html,css,output
There could be some overlap here with issue https://bugs.webkit.org/show_bug.cgi?id=161002
Note that Safari Tech Preview Release 20 appears to have fixed this issue.