2.2.46 [HTML] Section 17.12.1, Disabled controls
The specification states:
disabled [CI] When set for a form control, this boolean attribute disables the control for user input. When set, the disabled attribute has the following effects on an element: Disabled controls do not receive focus. Disabled controls are skipped in tabbing navigation. Disabled controls cannot be successful. The following elements support the disabled attribute: BUTTON, INPUT, OPTGROUP, OPTION, SELECT, and TEXTAREA. This attribute is inherited but local declarations override the inherited value.
All Document Modes (Internet Explorer 7)
Child elements (such as OPTION elements within a SELECT element) cannot disable themselves when their parent is enabled. In other words, the absence of the disabled attribute is inherited but cannot be overridden locally.
All Document Modes (Internet Explorer 8 and Internet Explorer 9)
The behavior differs from previous versions and modes insofar as child elements cannot override an inherited disabled attribute; if the parent is disabled, the child will be as well. However, child elements also inherit the absence of a disabled attribute (that is, the parent is enabled) and can override this by disabling themselves.