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.
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
https://codepen.io/memetican/pen/LYBrgqQ/9632bd83545b8fdf722b1b8b55e377f7?editors=1010
https://stackoverflow.com/a/21458090
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