HTML (Hyper Text Markup Language) and CSS (Cascading Style Sheets) are the most basic building blocks of web coding. Without these two things, you cannot create a website design and all you will end up with is plain unformatted text on the screen. You can't even add images to a page without HTML! The html is the document the css the design of the document
HTML has the DOM, and CSS has CSSOM, an acronym for CSS Object Model. When the browser is loading the HTML DOM, which is the HTML object tree, it finds a call to a CSS code and starts to build another tree, based on the CSS selectors. This tree makes a reference between the selectors, which will represent the elements in HTML and their properties visuals that will be modified by the CSS.