<!DOCTYPE html>
<html>
<head>
<title>The title of the document</title>
<style>
.message {
visibility: hidden;
}
#test:valid+.message {
visibility: visible;
}
</style>
</head>
<body>
<form>
<input type="text" placeholder="Type something..." id="test" required="required">
<span class="message">Some text</span>
</form>
</body>
</html>