I saw you have added the fix inside a media query – that’s probably why it doesn’t work for you in Safari on Windows:
@media all and (max-width: 768px) {
.teaser5c { width: 98%; min-height: 0 !important; }
.teaser5c .teaserpicture {width: 50%;}
#navibereich { position: relative !important; }
#main-menu {
border-radius:0;
-moz-border-radius: 0;
-webkit-border-radius: 0;
}
#main-menu ul,
#main-menu li {
-webkit-transform: translate3d(0px, 0px, 1px);
}
Move it out of the media query and it should work.