You can blame most forms’ poor etiquette on the way they’re built. Web forms that use a basic submit-and-refresh model of interactivity don’t respond until you hit the “submit” button—but it doesn’t have to be this way. Real-time inline validation can help people complete web forms more quickly and with less effort, fewer errors, and (surprise!) more satisfaction.

Inline validation gives people several types of real-time feedback: It can confirm an appropriate answer, suggest valid answers, and provide regular updates to help people stay within necessary limits. These bits of feedback can be presented before, during and / or after users provide answers.

https://alistapart.com/article/inline-validation-in-web-forms/