They may appear in any order. Attribute names are always case-insensitive.
Attribute values are generally case-insensitive.
A single character is used as the value of this attribute. A single, case-insensitive alphanumeric character from a browser's character set. The other options specify vertical alignment of text relative to the object on the same line. In some systems this is a class identifier. Browsers may use this value to skip over unsupported MIME types without needing to make a network access.