The Segoe UI font is not built into a browser. It is shipped with some Microsoft software. On a Mac, it is most probably not available, though a user might have acquired it separately or as part of some software package.
You should remove the @font-face rule and the font files uploaded onto a server. Segoe UI is protected by copyright, and it must not be used as a web font without permission from the copyright holder, Microsoft (which probably won’t give a permission).
You will need some other strategy, such as using another font either as the basic font or as a fallback font for visitors who don’t have Segoe UI in their systems. The Google font Open Sans is rather similar to Segoe UI, and it’s free (and it comes in 10 styles, including Book, Normal, Semi-Bold, Bold, and Extra-Bold weight).
Answer from Jukka K. Korpela on Stack Overflowhtml - Will Segoe UI work on Mac? - Stack Overflow
Segoe UI Font Support for Mac
Segoe UI or Open Sans for Business Email?
I need all latest Segoe UI fonts, can someone link me to them please
The Segoe UI font is not built into a browser. It is shipped with some Microsoft software. On a Mac, it is most probably not available, though a user might have acquired it separately or as part of some software package.
You should remove the @font-face rule and the font files uploaded onto a server. Segoe UI is protected by copyright, and it must not be used as a web font without permission from the copyright holder, Microsoft (which probably won’t give a permission).
You will need some other strategy, such as using another font either as the basic font or as a fallback font for visitors who don’t have Segoe UI in their systems. The Google font Open Sans is rather similar to Segoe UI, and it’s free (and it comes in 10 styles, including Book, Normal, Semi-Bold, Bold, and Extra-Bold weight).
It will work on any computer with Office 2007, 2010, Vista or 7:
The Segoe UI font family can be obtained as part of Microsoft Office 2007, Microsoft Office 2010, Windows Vista or Windows 7.
Will also work on XP if the user has downloaded a Windows Live package:
Segoe UI is installed into Windows XP if the user installs Windows Live Messenger, or Windows Live Mail
Source
For any computer that doesn't have the above, you will need to declare it with a @font-face