Cross-browser issue: using negative margins hides text in Safari only

I’ve created a landing page for a coming event, and my header text is partially obscured in Safari, but looks good in Firefox, Chrome, and IE. See this in action at http://www.litmos.com/dreamforce.

My header text “Visit booth W138 in Moscone West at Dreamforce” looks good on the black background in IE, Chrome, and Firefox, but in Safari the top 1/2 of the text is cut off. Please help.

Thank you,
Nancy