Inline Validation in Web Forms
http://www.alistapart.com/articles/inline-validation-in-web-forms/

Wroblewski, Luke
List Apart, A
2009

Abstract:

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.