mySVGRectElement.x in SVG 1.1

The location of the left side of the element.
This property is read-only.

Property Type
SVGAnimatedLength

As an SVGAnimatedLength object, getting this property and expecting a single number will fail, as would an attempt to set it to a value. For UAs that support this property, the proper method for setting the base value would be something like:

mySVGRectObject.x.baseVal.value=100