And I assume your client is using Safari and/or IE on a Mac. :~)
Mac browser issues, particularly with javascript and css, is a long standing problem. The issues seem to vary not only by browser version, but also by OS version.
In most cases, I’ve shown the client several different sources of web site visitor stats for Mac vs PC and have given him the choice of keeping the look as is, or redesigning the page layout to a format that will also work properly for that less than 5% that use a Mac.