If you use OpenGl blending functions, you probably are doing something with the alpha buffer (or not). By default the alpha buffer is disabled on the html / gwt / web config.
So you need to go to the HtmlLauncher and set the alpha boolean to true:
public GwtApplicationConfiguration getConfig() {
final GwtApplicationConfiguration config = new ...;
config.alpha = true;
return config;
}A side effect by enabling the alpha buffer is the actual background will become pseudo transparent. At the end of the drawing the alpha buffer will be handled as the alpha channel of the whole canvas. So if there is no need for some transparency, at the end of drawing the whole alpha buffer should be filled with 1s.
done_