Client-side validation can improve your app's usability and responsiveness by informing users of the expected format, length, or range of the input, and by highlighting any errors or warnings before submission. It can also reduce network traffic and server load by filtering out invalid or unnecessary input. However, client-side validation is not enough to ensure security, as it can be easily bypassed, modified, or disabled. Thus, you should never rely on client-side validation alone and always use server-side validation as well. To implement client-side validation for your mobile app, you can utilize various tools and techniques such as HTML attributes like type, pattern, required, min, max, or maxlength; CSS pseudo-classes like :valid, :invalid, :required or :optional; JavaScript functions like checkValidity(), setCustomValidity(), or reportValidity(); and JavaScript libraries/frameworks like jQuery, Angular, React, or Vue. An example of this is the code used to validate an email input using HTML, CSS and JavaScript: <input type="email" id="email" name="email" required><span id="error" hidden>Please enter a valid email address.</span><style>input:invalid {border: 2px solid red;}#error {color: red;}</style><script>var email = document.getElementById("email");var error = document.getElementById("error");email.addEventListener("input", function() {if (email.checkValidity()) {error.hidden = true;} else {error.hidden = false;}});</script>.