Required attribute for guided page variables

The default option in marketo variables is really useful, but I can see instances where I would not want to assume a default value.

It'd be great to be able to set a required flag on variable declarations.

<meta class="mktoColor" id="color" mktoName="My Color Variable" default="" required="required">

Adding to that idea, a placeholder attribute to suggest, but not set a default

<meta class="mktoColor" id="color" mktoName="My Color Variable" default="" required="required" placeholder="#0c0c0c">