How can you validate a form on the client side using HTML?

To validate a form on the client side using HTML, you can utilize the built-in form validation features that HTML provides. Here is an example using HTML5 attributes:

```html

function validateForm() {
var form = document.getElementById('myForm');
if (!form.checkValidity()) {
alert('Please fill out all required fields.');
return false;
}
return true;
}

```

In this example, the `required` attribute is used on the input fields to indicate that they must be filled out before the form can be submitted. The `checkValidity()` method is called on the form element in the `validateForm()` function to check if all the required fields are filled out. If validation fails, an alert is shown to prompt the user to fill out the required fields.