Added tabIndex option to viewer

src/navigator.js

@@ -98,6 +98,7 @@ $.Navigator = function( options ){
         sizeRatio:     $.DEFAULT_SETTINGS.navigatorSizeRatio
     }, options, {
         element:                this.element,
+        tabIndex:               -1, // No keyboard navigation, omit from tab order
         //These need to be overridden to prevent recursion since
         //the navigator is a viewer and a viewer has a navigator
         showNavigator:          false,

src/openseadragon.js

@@ -126,6 +126,10 @@
   *     The element to append the viewer's container element to. If not provided, the 'id' property must be provided.
   *     If both the element and id properties are specified, the viewer is appended to the element provided in the element property.
   *
+  * @property {Number} [tabIndex=0]
+  *     Tabbing order index to assign to the viewer element. Positive values are selected in increasing order. When tabIndex is 0 
+  *     source order is used. A negative value omits the viewer from the tabbing order.
+  *
   * @property {Array|String|Function|Object[]|Array[]|String[]|Function[]} [tileSources=null]
   *     As an Array, the tileSource can hold either Objects or mixed
   *     types of Arrays of Objects, Strings, or Functions. When a value is a String,

src/viewer.js

@@ -268,7 +268,7 @@ $.Viewer = function( options ) {
         style.left     = "0px";
     }(this.canvas.style));
     $.setElementTouchActionNone( this.canvas );
-    this.canvas.tabIndex = 0;
+    this.canvas.tabIndex = options.tabIndex || 0;
 
     //the container is created through applying the ControlDock constructor above
     this.container.className = "openseadragon-container";