The decoder-only version of the reference implementation (libflif_dec) uses the Apache 2.0 license (specifically for this reason, I'd assume). Browsers shouldn't need to encode FLIF images very often, so decoder-only would be fine for that use case.
Not even the most restrictive copyleft license hampers evaluation. You are literally free to do whatever you want with the program on your own hardware. Its only when you start redistributing that the license kicks in.
A web browser doesn't need to "evaluate the efficacy" of an image codec. It either supports it or it doesn't, and in the former case it only needs to decode, which means it only needs to incorporate Apache 2.0 licensed code.
Like I said: they shouldn't need to encode images very often, and that doesn't seem like all that much of a common use case (nor would it preclude a browser from implementing decoding and just not supporting FLIF for encoding-dependent functionality like that).
