HTML elements can be nested (elements can contain elements).

All HTML documents consist of nested HTML elements.

This example contains four HTML elements:

My First Heading
My first paragraph.

HTML elements with no content are called empty elements.

is an empty element without a closing tag (the
tag defines a line break).

Empty elements can be “closed” in the opening tag like this:
.

HTML5 does not require empty elements to be closed. But if you want stricter validation, or if you need to make your document readable by XML parsers, you must close all HTML elements properly.