responsive tip calculator using flex, grid, media and event listeners

I really need a feedback on how to activate validation message since there should be no submit button . M validation messages only come up on hover. Also I need a feedback on how to activate my calculator on change of my custom input, it only activates on click or when using the navigator. Tried using the 'change' event listeners on it but was not working so i just deleted it . Thanks . I wait in anticipation .