Relative path in external CSS file - relative to what?

The w3c spec states that relative paths in the CSS file are relative to the stylesheet, not the HTML file, as you might expect. But...
Date Created: 2nd January 2007
Date Modified: 2nd January 2007
Home > CSS > Relative CSS path

The following DIV has a class of "test_relative_path" defined in "/_styles/test_relative_path.css" - it sets a background-image, but the URL in the CSS file is a relative URL (img/wetlilac.gif). But relative to what?

[Test on the BODY] - Reqd for NN4

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