This is a warning. It will appear even if there is no error in the script. It just reminds developers that in HTML5 canvas, the timeline number starts from 0. In swf, the timeline number starts from 1.

For example: if you execute this.gotoAndStop(5) in html5 canvas, it will actually stop on the 4th frame of the timeline, but in swf, it will stop on the 5th frame of the timeline.