White space is not allowed, nor other markup. This sequence is reserved for use by the standardization forum. Tags can contain attributes. Attributes have to follow these rules. As already mentioned, all values must be quoted.
Attributes can only appear in start tags and empty element tags.
No attribute may appear more than once in the same start tag. All attributes must be declared in the DTD if present, and their value must be of the correct type. An attribute cannot contain a reference to an external entity. XML comments are written the same way as HTML comments. The text, including the markup, would be displayed.