It depends on where Office is being used. If it's on Windows Desktop, Office Add-ins use an Internet Explorer to load the hosted webpage in Office. For iOS, it will rely on the native WebView control. If on Office Online, it will use a sandboxed iframe, in which ever browser you open Office Online with.
If you're trying to determine the browser Office is running in, I believe sniffing the user agent string will work for you.
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