Elements may have associated properties, called attributes, which may have values (by default, or set by authors or scripts). Attribute/value pairs appear before the final ">" of an element's start tag. Any number of (legal) attribute value pairs, separated by spaces, may appear in an element's start tag. They may appear in any order.

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.

Boolean attributes
Some attributes play the role of boolean variables (e.g., the selected attribute for the OPTION element). Their appearance in the start tag of an element implies that the value of the attribute is "true". Their absence implies a value of "false".
Boolean attributes may legally take a single value: the name of the attribute itself (e.g., selected="selected").
This example defines the selected attribute to be a boolean attribute.

selected     (selected)  #IMPLIED  -- option is pre-selected --

The attribute is set to "true" by appearing in the element's start tag:

<OPTION selected="selected"> ...contents... </OPTION>

In HTML, boolean attributes may appear in minimized form -- the attribute's value appears alone in the element's start tag. Thus, selected may be set by writing:

<OPTION selected>

instead of:

<OPTION selected="selected">

Authors should be aware that many user agents only recognize the minimized form of boolean attributes and not the full form.