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:
<dict>
<key>monospace</key>
<string>Courier</string>
<key>standard</key>
<string>Times</string>
<key>sans-serif</key>
<string>Helvetica</string>
<key>serif</key>
<string>Times</string>
</dict>
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.