1.DOM（The Document Object Model）
A way to manipulate the structure and style of an HTML page.
It represents the internals of the page as the browser sees it, and allows the developer to alter it with JavaScript.
2.HTML is an XML-like structure
To access the DOM from JavaScript, the document object is used.
3.Getting an element
1)By ID
2)By Tag Name
document.getElementByTagName("a")
Returns a NodeList=an array of the DOM Elements
3)By Class Name
document.getElementByClassName
as TagName
4)By CSS Seclector
document.querySelector('#header'); =ID~~~
document.querySelectorAll(.btn);