[nikita.mmf]

function getChildElementsByTagName ( element, tagName ) {
	var children = [];
	tagName = tagName && tagName.toLowerCase();
	for ( var child = element.firstChild; child; child = child.nextSibling ) {
		if ( child.nodeType === 1 && (!tagName || child.nodeName.toLowerCase() === tagName) ) {
			children.push( child );
		}
	}
	return children;
}
alert(getChildElementsByTagName( document, 'html' ).length)
alert(getChildElementsByTagName( document.body, 'div' ).length)