Weird extensions from the '90s.

gistfile1.txt

HTML Components
http://msdn.microsoft.com/en-us/library/ms532146.aspx
http://www.w3.org/TR/NOTE-HTMLComponents

Element behaviors
http://msdn.microsoft.com/en-us/library/ms531426(v=vs.85).aspx

Introduction to DHTML Behaviors
http://msdn.microsoft.com/en-us/library/ms531079(v=vs.85).aspx

Custom Tags
http://msdn.microsoft.com/en-us/library/ms531076(v=vs.85).aspx

XBL
https://developer.mozilla.org/en-US/docs/XBL/XBL_1.0_Reference
http://www.w3.org/TR/xbl/

Widgets
http://dev.w3.org/2006/waf/widgets-land/