It does happen when the developer cares to do it; I know several that do. Website layouts need to be made linear (single column) for display on mobile and the

It does happen when the developer cares to do it; I know several that do. Website layouts need to be made linear (single column) for display on mobile and the content order usually reflects that anyway. They also need to make sense when using screen readers. Disabling CSS and checking the result gives you a quick sanity test that the content order makes sense and that the markup is used somewhat correctly. It won't be pretty, but it should be useable.