This is a test for image dimension, when the viewport width is given in percent. The DocBook imagedata Element states: the viewport area defines the space reserved in the flow of content for the image. The viewport area is specified by the width and depth attributes. Viewport area dimensions expressed as a percentage are a percentage of the available area. For example, a width of 50% when an implementation is rendering in a column 6in wide is equivalent to specifying a width of 3in. For this Document, we expect the with of the available area to be $nominal-page-width , which defaults to '6in'. Specifying the viewport area with @width='50%' should result in a viewport area width of 3 inch or 288 pixel (assuming 96 pixel per inch).