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floats being IEEE compliant is optional by the language standards. Otherwise the bit representation is not specified.

It still requires 0 to be represented as all bits set to zero.

Where does the ISO C standard say so?

edit:

https://cigix.me/c17#6.2.6.1.p1 seems relevant, I did not find anything for floating point types.

edit2:

Notes are probably not normative, but still:

That footnote confirms the traditional position that a floating point zero need NOT have an all-zero bit representation.