Test that no clamping occurs before interpolation

In the context of color, the spec encourages late clamping but does not
require it:
"User agents should clamp color values to allow color range values as
late as possible, but note that system differences might preclude
consistent behavior across different systems."
http://www.w3.org/TR/SVG/animate.html#AnimateColorElement

With opacity, nothing prevents us clamping late, after interpolation.
Use of late clamping has been observed on the public web.