Hosting the WebBrowser in your application makes it easy to create slick-looking textual UI’s. HTML is a fairly expressive markup language and the results can be very professional with little difficulty. The graphical side is a little less impressive. Thankfully, SVG and VML (native support in the WebBrowser control) go a long way to fill the gap. Both have capabilities that blow away good old Win32 GDI drawing. Even so, there may be times that you wish you could grab an HDC for the WebBrowser control and start drawing on it. The WebBrowser control actually has a system built into it that allows you to do just that.
IHTMLPainter is part of a system in MSHTML called Binary Behaviors. Behaviors can do lots of cool stuff. Painters are a special kind of behavior called a rendering behavior. IHTMLPainter::Draw is called by MSHTML before or after the HTML content is rendered allowing you to draw below or above the content. MSHTML passes an HDC into method that you can use with any GDI or GDI+ drawing function. Basically, behaviors are COM objects that you implement and are associated with HTML elements, either in the HTML itself using CSS or progammatically using addBehavior. The MSDN has a nice tutorial on creating a rendering behavior that includes sample code.