[susan53]

While/Whereas meet none of the criteria for co-ordinators mentioned above. Eg:
1) A co-ordinator cannot be preceded by another co-ordinator. While/whereas can : Dogs seem more intelligent than cats, but whereas dogs etc etc
2) Co-ordinators allow ellipsis of repeated items in the second clause, but subordinators don't. So : I love ironing but hate cleaning the house (I is repeated and therefore can be omitted), but you can't say : *I love ironing while hate cleaning the house - you must repeat the I : I love ironing while I hate cleaning the house.

There are other criteria (a good grammar will list them for you) - but while/whereas meet none of them. So, we have and, but, or as "pure" co-ordinators, while/whereas/although and many others as "pure" sub-ordinators, and other words (for, so, then) which appear to be in the middle - acting in some ways like co-ordinators and in some ways like subordinators. Notice though that these are syntactic criteria and not, as you say, semantic. Semantic criteria have nothing to do with the distinction. many expressions may be similar semantically but belong to completely different word/syntactic classes.