Usability means making interfaces that really speak to users; speak volumes by getting their attention.
Whenever we are getting ready to introduce elements that move or grow or appear or disappear, we always take into consideration non-sighted users. Be ready for disabled users.
Prepare a form element that has a message that follows the element in the DOM. Any error message can be written there, perhaps some message regarding validation of the element. Let us just whimsically play a name game to demonstrate.
<form action="" method="get"> <label for="name">Name: </label> <input type="text" name="name" value=""/> <span id="error_message"></span> <br/> <input type="button" ...