[basic.fundamental] p12 says

The value representation of floating-point types is implementation-defined.

For other integer types, we did not emphasize this point for them. Isn't the value representation for integer types also implementation-defined? Consider the impact for the value representation with the difference between big-endian and little-endian in a concrete environment, and assume the width for short int is always16.

short i = 2;
//  big-endian 00000000 00000010
// little-endian 00000010 00000000

The example wants to convey that saying the value representation is implementation-defined does not mean the range of representable values is implementation-defined too.

So, saying that the value representation of floating-point types is implementation-defined is not all that important here. It is not helpful for interpreting whether a floating-pointer type can represent positive infinity or not. As said in #5407 (comment), we consider the existence of these values for a floating-pointer type is implementation-defined, which means that we care more about the range of representable values. I think, change the above rule to that

The range of representable values for floating-pointer types is implementation-defined.

which is more helpful.