Defining an XML format for a 2D array (Grid) of items

I need to define an XML format and then read it in ActionScript3, which will be storing:

• the number of rows and columns in a grid
• the horizontal and vertical spacing in pixels
• the size of each square in the grid in pixels
• an optional label for each square in the grid
• an optional hyperlink for each square in the grid

The following is the kind of thing that would meet my needs at the moment, which demonstrates the four square types:

<?xml version="1.0" encoding="utf-8"?>
<grid columns="2" rows="2" horizontalSpacing="10" verticalSpacing="10" 
      squareWidth="300" squareHeight="300">
    <column>
        <square label="Square(1,1)" url="http://example.com/1/1/" />
        <square label="Square(1,2)" />
    </column>
    <column>
        <square url="http://example.com/2/2/" />
        <square />
    </column>
</grid>

My questions are things akin to "Should there be units on the spacing and size? (like 10px or 300px)" and "How bad would it be to use a XHTML subset of (table, td, tr with a elements in each cell)."

But really I'm fishing for any precedent, ideas, or best practices. Particularly as they are relevant to processing such a file in ActionScript3. The goal is to be able to read the grid properties and to define a function that will give back a square's label and url given a row and column. (The easy thing about this format is that getting a square is just a matter of indexing into children of the grid node by integer... first column, then row.)

Couldn't you use JSON instead? It seems more adapted for this sort of task