Document: rootElement property

Document.rootElement returns the Element
that is the root element of the document if it is an
svg element, otherwise 
null. It is deprecated in favor of
Document.documentElement, which returns the root element for all
documents.
Value
For SVG elements, the Element that is the root element of the document; otherwise 
null.
If the document is a non-empty SVG document, then the 
rootElement will be
an SVGSVGElement, identical to the 
documentElement.
Specifications
Browser compatibility