The default fonts for WebKit/Safari under Mac OS X are as follows:
- Monospace -> Courier
- Sans-serif -> Helvetica
- Serif -> Times
- Standard (default) -> Times
This is gleaned directly from the WebKit source, specifically the XML/plist file at
Source/WebCore/Resources/DefaultFont.plist.in that's available online here. It includes the following lines:
The same file also includes the default monospace, sans-serif, and serif fonts for Korean, Japanese, Simplified Chinese, and Traditional Chinese, if those are of any interest to you.