window.openDialog() is an extension to window.open(). It behaves the same, except that it can optionally take one or more parameters past windowFeatures, and windowFeatures itself is treated a little differently.
Returns an array-like object of all child elements which have all of the given class names. When called on the document object, the complete document is searched, including the root node. You may also call getElementsByClassName() on any element; it will return only elements which are descendants of the specified root element with the given class names.
insertAdjacentHTML() parses the specified text as HTML or XML and inserts the resulting nodes into the DOM tree at a specified position. It does not reparse the element it is being used on and thus it does not corrupt the existing elements inside the element. This, and avoiding the extra step of serialization make it much faster than direct innerHTML manipulation.