limitations of units and a proposed "percent+offset" extension

Currently, SVG units can express a coordinate N pixels from the top or left
edge of the viewport, but not from the right or bottom edge (unless N is 0,
in which case I can use %). So it's easy to draw a 10px-wide rectangle along
the left edge of the viewport, but not the right edge.

I can work around this limitation in some cases by introducing an <svg>
container. For example, I can actually draw a 10px wide rectangle along the
right edge using
<svg ...>
  <svg x="100%" style="overflow:visible">
    <rect x="-10" width="10" y="0" height="100%"/>
  </svg>
</svg>
However, as far as I can tell there is no way to use this to draw, say, a
rectangle inset from the viewport by 10px on each side. (And of course it's
very ugly.)

A very simple extension would be to allow length values like "50%-10". This
is necessarily trivial to implement since length interpretation already
requires support for viewport percentages and user-space units; this just
combines them into a single value.

An important use-case for this is "nine-grid" image stretching (see e.g.
http://msdn.microsoft.com/en-us/library/bb189722.aspx). It would be very
useful if SVG images supported this, for example for use as background
images for flexible HTML content.

Of course, SVG direly needs a real layout mechanism. But this is a very
small tweak that would provide a great deal of value.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]