Void elements do not have closing tags, they cannot have any content and only a start tag, you include attributes

There are several common boolean attributes you will encounter in HTML, such as disabled, readonly, and required. These attributes are used to specify the state of an element, such as whether it is disabled, read-only, or required.

In this example, the preconnect value for the rel attribute tells the browser to create an early connection to the value specified in the href attribute. This is done to speed up loading times for these external resources.