Disable warning message in Safari 5.1.2
This error message is not coming from Safari.  It's coming from the content management system of the website: the SLAED CMS, in particular from the editor this CMS uses, TinyMCE.  There isn't anything you can do.  The website author needs to change this setting on their site or upgrade the CMS/TinyMCE version.  The site you refer to is running a version of TinyMCE from 2005 when Safari support was new.