whole document cloning not working in javascript

Question

I am working on some stuff meanwhile i get into something which i didn't understand core reason behind it.

my idea(good or bad) is to clone a document and add changes to it and then reassign my cloned object into document

var tu=document.clone(true);
 getComputedStyles(document)//returns all current css values
tu.getElementsByTagName("body")[0].style.backgroundColor="yellow";//when i print tu and inspect style is visible on body
tu.getElementsByTagName("body")[0].style.width="100px";
getComputedStyles(tu)//returns all empty values.

document=tu;

But every step return without an error but after assigning tu to document my body bgColor is still white.

one more thing why getComputedStyles() are returning empty values?

Answer 1

Finally i got something which might be usefull

var documentClone=document.clone(true);
documentClone.body.style.background="yellow";
documentClone.body.style.background="red";`\\many number of operations`


//after many changes
document.replaceChild(documentClone.documentElement,document.documentElement);
//now you can find all your changes..

Answer 2

window.document是不可写的。