HTML handles data validation for forms through attributes that specify the type of input, the pattern or the minimal amount of characters we're expecting an user to enter on an specific field.

<input type="text" name="username" pattern="[A-Za-z]{5,9}" required>

And we could simultaneously specify other input validation handlers using JavaScript.

document.querySelector('form');.addEventListener('click', (event) => {
    const username = document.querySelector("input[name='username']")
    if (username.value.length < 5) {
        event.preventDefault(); 
        alert("Username must be at least 5 characters long.");
    }
});

The code above handles the submission of form input data by preventing the default behavior of the submission event until the username input is entered by a user with more than 5 characters.

It's beneficial to use both of these methods when validating form inputs because both offer different tools that when combined give us a more secure and dynamic way of validating form data.