I put HTML tags in uppercase, XML tags in lower case. XML is case sensitive. XML is case-sensitive. 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.