As far as page layout goes progressive should not behave any different than non-progressive since images have an intrinsic size in the header which is considere

As far as page layout goes progressive should not behave any different than non-progressive since images have an intrinsic size in the header which is considered by the rendering engines when sizing the image layer. It does not change throughout the load process. Multiplexing will load the necessary headers just as soon with non-progressive images.