In the Structuring content with HTML module, we covered what HTML is and how it is used to mark up documents. These documents will be readable in a web browser. Headings will look larger than regular text, paragraphs break onto a new line and have space between them. Links are colored and underlined to distinguish them from the rest of the text.
What you are seeing are the browser's default styles — very basic styling that the browser applies to HTML to make sure that the page will be basically readable even if no explicit styling is specified by the author of the page. These styles are defined in default CSS stylesheets contained within the browser — they have nothing to do with HTML.
The web would be a boring place if all websites looked like that. This is why you need to learn about CSS.