In the example given on link http://dev.w3.org/2006/webapi/WebIDL/#es-sequence there the example illustrates the ECMAScript implementation.
Here

// This evaluates to false.
canvas.getSupportedImageCodecs() == canvas.getSupportedImageCodecs();

Should be 
// This evaluates to true.
canvas.getSupportedImageCodecs()[0] == canvas.getSupportedImageCodecs();

Else
// This evaluates to false.
supportedImageCodecs[0] == canvas.getSupportedImageCodecs();