Required input fields

The simplest way of marking an input as required (and the only pure HTML way) is to simply add the "required" attribute to the input element like this:

<label for="name">Name:</label>
<input type="text" id="name" name="user_name" required>

<label for="mail">Email:</label>
 <input type="email" id="mail" name="user_email" required>

<label for="password">Password:</label>
<input type="password" id="password" name="user_password" required>

The caveat to the required attribute is that it only works if the browser you use support it. All relatively modern browsers do, but you can't always be sure your users will be using a modern browser.

The other thing worth mentioning is that while this will warn users when they forget to fill something out it won't stop malicious users from submitting the form empty. As with all client side checking it can be disabled by the client. So you will still have to have checks on the server that receives data from the form, in order to verify that data was actually entered even if you mark fields as required.

Thank you for the answers! I appreciate the explanation that further action may need to be taken outside of the HTML to ensure that the form is not submitted empty.