I'm trying to figure out an alternative way to implement calc using vw units, as this isn't supported yet in Safari/mobSafari (probably IE too when I get to that). I've set up a JSfiddle to show a simple example of how it differs for Safari. I've had a bash (unsuccessfully) with JS but with no success. Any pointers in the right direction appreciated - if there's a possible alternative via CSS that would be perfect.
It's basically recreating left: calc(-50vw + 50%)
in a cross-browser supported way.