Comment 1 for bug 171088

What you're actually asking for (hence the name change for
this feature request) is a way of specifying constraints in SVG.

A member of my research group has investigated and proposed
an extension to SVG that allows specifying constraints like
you suggest. His page about it can be found here:
http://www.csse.monash.edu.au/~clm/csvg/index.html

I myself am interested in the use of constraints for
specifying interactive layout tools for things like
alignment and distribution using constraints. I have
implemented these tools in a prototype diagram editor named
Dunnart:
http://www.csse.monash.edu.au/~mwybrow/dunnart/

What's not clear is the interface that should be offered to
the user for creating and interacting with such constraints.
 Both Cameron and I are currently carrying out usability
studies looking at this question.

The plan is to add some of this stuff to Inkscape
eventually. It will happen, we just don't have a date yet.