Today's message is: <span data-bind="text: myMessage"></span>
<script type="text/javascript" src="~/Scripts/knockout-2.3.0.debug.js"></script>  
<script type="text/javascript">   
    var viewModel = {       
        myMessage: ko.observable()    
    };
    viewModel.myMessage("Hello, world!");
    ko.applyBindings(viewModel);
</script>

price: ko.observable(24.95)
    viewModel.priceRating = ko.dependentObservable(
        function () {
         return this.price() > 50 ? "expensive" : "affordable";
        }, viewModel);

The item is <span data-bind="text: priceRating"></span> today.

viewModel.myMessage("<i>Hello, world!</i>");