They may appear in any order. Attribute names are always case-insensitive.
Attribute values are generally case-insensitive.
It overrides the default behavior of placing new layers on top of all existing layers. References to layers not yet defined result in default layer creation behavior - i.e. the new layer will be placed on top of all existing layers. An existing valid alphanumeric layer NAME value. Default behavior for a layer is to be transparent. It overrides the default behavior of placing new layers on top of all existing layers.