example: http://jsfiddle.net/gbJZE/5/

.attr method contains:

#!div style="font-size: 100%"
Code highlighting:
  {{{#!python
  if ( notxml ) {
			name = name.toLowerCase();
			hooks = jQuery.attrHooks[ name ] || ( rboolean.test( name ) ? boolHook : nodeHook );
		}
//rboolean.test( name ) = true -->hooks = boolHook 
  

}}}

than the boolHook method is called:

#!div style="font-size: 100%"
Code highlighting:
  {{{#!python
boolHook = {
	get: function( elem, name ) {
		// Align boolean attributes with corresponding properties
		// Fall back to attribute presence where some booleans are not supported
		var attrNode,
			property = jQuery.prop( elem, name );
		return property === true || typeof property !== "boolean" && ( attrNode = elem.getAttributeNode(name) ) && attrNode.nodeValue !== false ?
			name.toLowerCase() :
			undefined;
	},
//method returns the name.toLowerCase()
  

}}}

May be it should return boolean value?

If not may be it should be pointed in documentation to .attr method http://api.jquery.com/attr/ ?