JSFiddle - React, Tailwind, and code Playground

HTML

<p>This doesn't work!</p>
<div data-bind="html: extraHTML, value: selectedExtra"></div>
<span data-bind="text: selectedExtra"></span>
<p>selectedExtra Value: <span data-bind="text: selectedExtra"></span></p>
<p>This does work!</p>
<input type='number' data-bind="value: selectedExtra2, valueUpdate: 'input'">
<p>selectedExtra2 Value: <span data-bind="text: selectedExtra2"></span></p>

JavaScript

var ViewModel = function () {
    this.extraHTML = ko.observable("<input type='number' data-bind='value: selectedExtra, valueUpdate: 'input''/>");
    this.selectedExtra = ko.observable();
    this.selectedExtra2 = ko.observable();
}

ko.applyBindings(new ViewModel());