IE11 Bugfix (boxes were not centered in widescreen mode, they were left aligned.)
IE10 Bugfix regarding alignment of content boxes (the justify-content: center for oldSafari is bad for IE10; So I added the prefixed version -webkit-box-pack for oldSafari and every other browser doesn't get the property at all.)
Fix for browsers that support media queries, but not flexbox (=IE9). Therefore I added Modernizr.