>SGML and XML are for text I'd say anything which can be nested. For example, XML makes sense for a layout engine: Arbitrary amounts of buttons can fit insid

>SGML and XML are for text I'd say anything which can be nested. For example, XML makes sense for a layout engine: Arbitrary amounts of buttons can fit inside a layout? Make it a child. Text in a button? If only one and can't have children tags? Make it an attribute. Else -> Make a child.