By default, OpenLayers draws, or renders, the map in the browser using an HTML 5 canvas element and its associated 2D drawing context. This is known as the Canvas renderer. However, OpenLayers actually comes with three renderers. The ability to choose a different renderer for the map can be very powerful but it should also be used with care. Each renderer has some specific capabilities and limitations. In particular, only the Canvas renderer is considered stable at the time of writing this book.
The Canvas renderer draws the map's contents onto an HTML 5 canvas element (http://en.wikipedia.org/wiki/Canvas_element). The canvas element is a high-performance 2D drawing surface supported by all modern web browsers (on Internet Explorer, Canvas support starts with version 9). The canvas renderer is the most fully supported renderer in OpenLayers. It does not support some of the advanced features of the WebGL renderer, including 3D, controlling the contrast, brightness...