We have a definition of what we consider a valid programming language for answers on PPCG. (If you disagree with this definition, please do so on that other post, and not here.)
The one type of challenge where people regularly tend to ignore this is for challenges with fixed output, i.e. mostly kolmogorov-complexity. These sometimes get answers, which are simply a compressed version of the resulting image, or which are written in HTML + CSS only (which don't fulfil the criteria for being a programming language either).
Some examples (all are graphical output - let me know if you can think of examples in other contexts):
Especially when providing just an image file, the question also arises if that isn't a loophole of using built-in compression.
So, should we require answers to such challenges also to be written in programming languages, or are we more lenient here?
(Note, if we decide that we do only want answers in programming languages, every challenge author who does not like that would of course be allowed to override that decision in their individual challenges. This is only supposed to be a default.)