Actually, C standard does not directly specify that. Only fact about structure layout that is explicitly required by standard is that address of first field is same as address of the structure itself, other constraints on structure layout can be deduced from other requirements scattered through the standard, but nothing in standard requires in memory field order to be same as declaration order.
"A structure type describes a sequentially allocated nonempty set of member objects (and, in certain circumstances, an incomplete array), each of which has an optionally specified name and possibly distinct type"
I think sequentially allocated states that fact. From what information do you mean it can be deduced?
ISO C actually explicitly specifies that the order is same in §6.7.2.1.13.
The "deduced" remark was essentially intended to cover constraints that come from 6.2.6.1.2 ("...objects are composed of contiguous sequences of one or more bytes..."), ie. fields cannot be (conceptually) outside &foo and ((char*)&foo)+sizeof(foo), which can be hard to implement on sufficiently weird architectures.
