HTML doesn’t describe the layout of the page. It describes the relative structure of the content like which is the title, which is the heading, which is the sub-heading and which is the paragraph. How this content has to be shown and rendered will be decided by styling and it is called style sheets. In one way you can say that HTML is the flesh and bones of a person and CSS is the clothing on the person that overrides the default skin. Cascading Style Sheets tell browsers how to display HTML tags. These style rules override the default display rules that browsers have for HTML tags.
Hope this is useful, thank you.