2.2.47 [HTML] Section 17.12.2, Read-only controls
The specification states:
When set, the readonly attribute has the following effects on an element: Read-only elements receive focus but cannot be modified by the user. Read-only elements are included in tabbing navigation. Read-only elements may be successful.
All Document Modes (Internet Explorer 7)
While read-only controls are included in tabbing navigation and can receive focus, the user interface does not indicate that the control is read-only.