Void elements

HTML 4.01/XHTML 1.0 Strict includes the following void elements:

• area - clickable, defined area in an image
• base - specifies a base URL from which all links base
• br - line break
• col - column in a table [deprecated]
• hr - horizontal rule (line)
• img - image
• input - field where users enter data
• link - links an external resource to the document
• meta - provides information about the document
• param - defines parameters for plugins

HTML 5 standards include all non-deprecated tags from the previous list and

• command - represents a command users can invoke [obsolete]
• keygen - facilitates public key generation for web certificates [deprecated]
• source - specifies media sources for picture, audio, and video elements

The example below does not include void elements:

<div>
    <a href="http://stackoverflow.com/">
        <h3>Click here to visit <i>Stack Overflow!</i></h3>
    </a>
    <button onclick="alert('Hello!');">Say Hello!</button>
    <p>My favorite language is <b>HTML</b>. Here are my others:</p>
    <ol>
        <li>CSS</li>
        <li>JavaScript</li>
        <li>PHP</li>
    </ol>
</div>

Notice how every element has an opening tag, a closing tag, and text or other elements inside the opening and closing tags. Void tags however, are shown in the example below:

<img src="https://cdn.sstatic.net/Sites/stackoverflow/company/img/logos/so/so-icon.png" />
<br>
<hr>
<input type="number" placeholder="Enter your favorite number">

With the exception of the img tag, all of these void elements have only an opening tag. The img tag, unlike any other tag, has a self closing / before the greater than sign of the opening tag. It is best practice to have a space before the slash.