New in 2012r2
WebCanvas controls are very useful for implementing your own graphical controls because you can use the WebGraphics class to draw inside the WebCanvas region.
Remember: Browsers maintain an uncompressed raster image of the contents of the canvas in memory so it can be displayed to the user. A 1024x1024 pixel canvas requires 4MB of RAM (4 bytes of color data per pixel). A browser may refuse a request to create a canvas that is too large. For instance iOS5 has a memory limit of 10MB for a single web page (it has been increased to 25MB in iOS6). On a Retina display, canvases require 4 times as much memory as a non-retina display.
Unlike the DesktopCanvas, the WebCanvas cannot act as a parent control to other web controls.
WebGraphics, Graphics classes; UserGuide:Web Canvas topic