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VanadiumJS offers pretty slick JavaScript form validation. It allows you to specify how to validate an input field using the field’s CSS class:

<input class=":integer" type="text"/>
<input class=":min_length;4 :max_length;8" type="text"/>
<input class=":required" id="password" type="password"/>
<input class=":same_as;password" type="password"/>

It pains me to see class co-opted so, but I love the idea behind this. It also supports hooks for triggering AJAX server-side validation:

<input id="username" class=":ajax;/user/check_username.json" type="text"/>

… and the server just response with some simple JSON.