Pass-through attributes cannot be "overridden" directly from the tag’s Properties dialog. Any pass-through name containing a dash is considered an override. For example, "OnClick" is an allowed pass-through because it sets an attribute, and "Button-Text" is allowed because it overrides a Property, but "Button-OnClick" is not permitted because it overrides an Attribute.
However, a special set of "wrapper" Properties (not Attributes) can be overridden. Overriding a wrapper Property is equivalent at runtime to overriding the Attribute itself.
Since only public Properties (not Attributes) of a control can be overridden across User Control (panel) boundaries, a control's "wrapped" Attributes can be set declaratively (using pass-through attributes), even across User Control (panel) boundaries. For example, a specific page’s Menu's MenuItem's LinkButton's "onBlur" Attribute can be overridden by setting a pass-through similar to the following on the Page's Menu component/tag:
<MenuItem tag name>-Button-HtmlAttributes-onBlur
Trying to set the Attribute directly using "<MenuItem tag name>-Button-onBlur" would have no effect on the LinkButton, because LinkButtons do not have a Public Property named "onBlur".
You can customize the behavior and appearance of paneled controls (e.g. Buttons, MenuItems, and Pagination panel contents) from outside the panel without code customization. From the tag’s Properties dialog, for example, you can:
Make a button or hyperlink display a confirmation popup when clicked.
Make a control display text in Internet Explorer’s status area when the user hovers over the control.
Add advanced client-side behavior to buttons and other controls by overriding the Javascript event handler attributes such as onClick, onFocus, OnBlur, onMouseMove, onKeyDown, etc.