Oh right, why didn't I think of that? :-)
It's a good idea on images since using pixels or EMs for image widths breaks ADE---when the image is wider than the screen.
However, it doesn't seem to be working for tables. I tried setting width:30% to a table that's a child of a DIV that takes up the whole screen. The table is not 30% of that DIV. No other widths in the table are set