On INPUT elements, setting the title attribute typically allows you to add a message on top of the browser's own validation error message.
If you want to go deeper than that, you can use scripting with HTML5's setCustomValidity API.
HTMLObjectElement.setCustomValidity
This API allows you to handle validation errors with a custom message. Since an INPUT can fail multiple ways simultaneously, HTML5 encapsulates this in an event object;
https://developer.mozilla.org/en-US/docs/Web/API/HTMLObjectElement/setCustomValidity
https://udn.realityripple.com/docs/Web/API/HTMLObjectElement/setCustomValidity
Demo
https://codepen.io/memetican/pen/LYBrgqQ/9632bd83545b8fdf722b1b8b55e377f7?editors=1010
Triggering Validation without Submit
https://stackoverflow.com/a/21458090
Additional References
https://angelika.me/2020/02/01/custom-error-messages-for-html5-form-validation/
https://documentation.unbounce.com/hc/en-us/articles/360001198903-Modifying-Form-Validation-Error-Messages-in-the-Classic-Builder
https://stackoverflow.com/questions/5272433/html5-form-required-attribute-set-custom-validation-message
FAQs
Answers to frequently asked questions.