addEventListener does not function in Internet Explorer, therefore this code creates an error in IE.

By adding validation and using a browser appropriate mechanism ensures the code executes in all browsers.

While this may not seem like an issue with the library on its own, as IE/Windows does not yet implement Retina class displays, it is a problem when one concatenates and minifies all javascript libraries as IE fails to execute any code subsequent to the error, as experienced in my original testing.  I also prefer on principle that code should not create errors, even when those errors do not impact user experience.

This solution ensures that even in the future if IE and Windows start to support Retina class displays the library won't need further modification.

C.