They may appear in any order. Single quote marks can be included within the attribute value when the value is delimited by double quote marks, and vice versa. In certain cases, it is possible in HTML to specify the value of an attribute without any quotation marks. We suggest using quotation marks even when it is possible to eliminate them. Attribute names are always case-insensitive.
Attribute values are generally case-insensitive.
The definition of each attribute in the reference manual indicates whether its value is case-insensitive. HTML documents may compress better if you use lower case letters for element and attribute names. The reason is that the compression algorithms do a better job for more frequently repeated patterns, and lower case letters are more frequent than upper case ones. Comments must not be rendered by user agents as part of a document. This specification presents pertinent fragments of the DTD each time an element or attribute is defined.