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HTML describes the structure of a web page so the browser can turn text into a document with headings, paragraphs, links, lists, and other parts. When you load a page, the browser reads the HTML top to bottom, builds an internal tree of elements, then renders that structure into something you can see and interact with. Styling usually comes from CSS, and interactivity usually comes from JavaScript, but HTML is the base that both of those attach to.
To keep that separation clear, start by comparing what the browser renders with the markup that produced it.
The same visible page can be explained by different tags, but each tag has a specific meaning. A paragraph appears because the HTML contains a p element, and a link is clickable because an a element tells the browser there is a navigation target. If you only change CSS, the content stays the same even though it looks different, because HTML controls what the content is.
HTML is built from elements, which are the named pieces like p and a that the browser understands. Most elements are written using a start tag like <p> and an end tag like </p>, with content in between. Some elements are empty elements and do not wrap content, such as img, which represents an image by pointing to a file.
Elements can include attributes, which add extra information to an element, written inside the start tag. An attribute has a value, like href="https://example.com", and the browser uses that value to decide behavior, such as where a link goes. Elements can also be nested, meaning one element sits inside another, and that parent child relationship affects structure and meaning.
Nesting rule
If you forget an end tag or overlap tags, the browser guesses your intent and the rendered result can shift in surprising ways.
A full HTML page starts with <!doctype html>, which tells the browser to use modern standards mode. The outer html element wraps the whole document, while head holds metadata the browser uses and body holds the content the browser renders into the page. The title element lives in head and controls the tab text, not the visible page content.