Validation refers to the process of making sure the submitted data matches our expectations. Validation can be done client-side or server-side. For example, we can use the built-in HTML form validation properties to enforce rules, like a number that must be positive:

<input type="number" min="0" name="Age" required>

If a user attempts to submit a form containing this input is submitted, and the value is less than 0, the browser will display an error message instead of submitting. In addition, the psuedo-css class :invalid will be applied to the element.

We can also mark inputs as required using the required attribute. The browser will refuse to submit the form until all required inputs are completed. Inputs with a required attribute also receive the :required pseudo-class, allowing you to assign specific styles to them.

You can read more about HTML Form validation on MDN.

Client-side validation is a good idea, because is minimizes invalid requests against our web application. However, we cannot always depend on it, so we also need to implement server-side validation. We can write custom logic for doing this, but many web application frameworks also have built-in support for validation.