How can I add a custom onclick-handler to my SPAN-tags - like the Skype addon does?

When I look at the source code for a web开发者_JS百科page that has been rendered by Skype's Internet Explorer add on, I find that their custom html-element is a SPAN element with a specific attribute, "skypeaction" that is set to "dropdown" - this is, I suppose, to be able to open a dropdown menu when the user clicks on that element.

Apart from this, there is no javascript or anything that I can see that actually makes the dropdown menu pop up. I do know that the CSS code for the popup is an embedded reference in the skypeplugin dll, but I do not understand how this element is clickable!

Does someone here know how this might work, and if it's possible for me to replicate this in some way?

Skype installs a BHO and a helper extension of some sort that does the handling of the click in native code. Probably by doing something like this.

If you go to Tools -> Manage Add-ons in IE you can see they have a BHO and another helper extension that are registered.