Started adding links for Canvas and its 2D and WebGL contexts.

third_party/WebKit/Source/SpecMapping.md

Index: third_party/WebKit/Source/SpecMapping.md
diff --git a/third_party/WebKit/Source/SpecMapping.md b/third_party/WebKit/Source/SpecMapping.md
index 91f219fcd91510e925bd9063eef2c242b69a58d6..ea90bcc472d87f6d43bbd54aadec14485e0f7159 100644
--- a/third_party/WebKit/Source/SpecMapping.md
+++ b/third_party/WebKit/Source/SpecMapping.md
@@ -2,7 +2,7 @@
 
 Blink is Chromium's implementation of the open web platform. This document
 attempts to map terms and concepts found in the specification of the open web
-platfrom to classes and files found in Blink's source tree.
+platform to classes and files found in Blink's source tree.
 
 [TOC]
 
@@ -28,3 +28,27 @@ interface where the main implementation is
 
 The WindowProxy is part of the bindings implemented by a class of the [same
 name](https://cs.chromium.org/Source/bindings/core/v8/WindowProxy.h).
+
+### [canvas](https://html.spec.whatwg.org/multipage/scripting.html#the-canvas-element)
+
+An HTML element into which drawing can be performed imperatively via
+JavaScript. Multiple
+[context types](https://html.spec.whatwg.org/multipage/scripting.html#dom-canvas-getcontext)
+are supported for different use cases.
+
+The main element's sources are in
+[HTMLCanvasElement](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/html/HTMLCanvasElement.h). Contexts
+are implemented via modules. The top-level module is
+[HTMLCanvasElementModule](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.h).
+
+The
+[2D canvas context](https://html.spec.whatwg.org/multipage/scripting.html#canvasrenderingcontext2d)
+is implemented in
+[modules/canvas2d](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/canvas2d/).
+
+The
+[WebGL 1.0](https://www.khronos.org/registry/webgl/specs/latest/1.0/)
+and
+[WebGL 2.0](https://www.khronos.org/registry/webgl/specs/latest/2.0/)
+contexts ([Github repo](https://github.com/KhronosGroup/WebGL)) are
+implemented in [modules/webgl](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/webgl/).