I struggled with this for a couple minutes and found that this question hadn’t really been covered since the new binding context variables (like $index)had been introduced in knockout 2.1
The mistake I was making was that I simply forgot that $index itself is an observable, and must be unwrapped if we are using it in an expression in the data-bind attribute. ie,
<li class="row" data-bind="css: { alt: $index%2 }"></li>
should become
<li class="row" data-bind="css: { alt: $index()%2 }"></li>
woops 🙂