If you have a radial gradient with a start radius (fr) of 0, end radius (r) of 0 and corresponding points have the same coordinates (i.e <cx,cy,r> == <fx,fy,fr>), it could be said that the circles are both fully overlapping and that the focal point is on the edge of the end circle (if a degenerate circle can be said to have an edge that is).

The "fully overlapping" rule comes from [1]. The "focal point on the edge" rule doesn't appear to be that well-defined (at least I only see it in the annotations and figures in [1] and sections above mention it but only for spreadMethod=repeat).

Based on a simple test [2], there is currently some different interpretations: Gecko renders that test with yellow while Blink renders it blue.

[1] https://www.w3.org/TR/SVG2/pservers.html#RadialGradientNotes
[2] https://jsfiddle.net/0v3tuoq4/