Why is manipulating the DOM hard?

Every browser has its own JavaScript rendering engine and it has its own DOM layout engine. TheW3C standardized the DOM in 1998, but browsers have been slow to provide uniform support of theW3C standards. Microsoft’s IE uses a DOM layout engine called Trident (or MSHTML), all Mozillabased browers (including Firefox and Netscape) use the open source Gecko layout engine, Safari usesApple’s WebCore, and Opera uses its own proprietary Presto layout engine. All engines support DOM1.0, but support for the later DOM 2.0 and 3.0 is still spotty.