Flash defaults to "non-web-friendly" (windowed) flags by default, HTML5 defaults to "web-friendly" flags. As a commenter said, you can also make the canvas tag

Flash defaults to "non-web-friendly" (windowed) flags by default, HTML5 defaults to "web-friendly" flags. As a commenter said, you can also make the canvas tag opaque by using a special browser-specific style. If you remove the windowless flag from Flash, you would have to add the opaque flag to HTML5 to make it a fair comparison.