DOM and DOMImpl

[wahaha]

there are two classes DOM and DOMImpl,looks the same,what is the
difference between their functions?

[wahaha]

DOM.getAbsoluteLeft(Element element) calles elem.getAbsoluteLeft(),
and elem.getAbsoluteLeft() calles DOMImpl.impl.getAbsoluteLeft(Element
element).
why designed it like this?

[Thomas Broyer]

Legacy.

Originally, JavaScriptObject were opaque handles, there was no "overlay types". So DOM directly called DOMImpl (or DOMImplIE6, DOMImplMozilla, etc. using deferred binding). Then overlay types were introduced, and the com.google.gwt.dom.*, so part of DOMImpl were moved there, and DOM was changed to call the element's methods (so it doesn't directly depends on com.google.gwt.dom.client.impl.*)