Some tests in the CSS test suites require special fonts to be installed. They are available for download here.
A zipfile of all CSS test suite fonts is also available.
The Ahem font is used in many layout tests. Its glyphs are solid rectangles, which simplifies the testing of layout features that interact with text.
The CSSTest group of fonts is used in tests for CSS's font selection features.
The Ahem Extra group of fonts is another set used in some CSS2.1 tests for CSS font selection features.