See

To see the DOM in a browser, you use the devtools. The DOM is represented in the Elements panel.

Example with Chrome, on how to check an element:

• Hit F12 to open the devtools
• Select the Select icon
• Select an element on the page
• The element as seen by the DOM can then be seen in the Elements panel of the devtools

Modification

• JavaScript may add, delete, or change DOM nodes:
  • with pure Web Api Dom call
  • or with a DOM library such as Jquery
• CSS can change the DOM, too, via the content property.

Watcher

See How to use MutationObserver to track mutation on the DOM ?