The nominal image width
Graphic widths expressed as a percentage are problematic. In the
following discussion, we speak of width and contentwidth, but
the same issues apply to depth and contentdepth.
A width of 50% means "half of the available space for the image."
That's fine. But note that in HTML, this is a dynamic property and
the image size will vary if the browser window is resized.
A contentwidth of 50% means "half of the actual image width".
But what does that mean if the stylesheets cannot assess the image's
actual size? Treating this as a width of 50% is one possibility, but
it produces behavior (dynamic scaling) that seems entirely out of
character with the meaning.
Instead, the stylesheets define a
nominal.image.width and convert percentages to
actual values based on that nominal size.