JSFiddle - React, Tailwind, and code Playground

JavaScript

Ext.create('Ext.container.Container', {
    renderTo: Ext.getBody(),
    items: [
        {
            xtype: 'button',
            text: 'start listening',
            handler: function() {
                var domContainer = this.up().getComponent('htmlContainer').el.dom;
                if (domContainer.addEventListener) {
                    domContainer.addEventListener('DOMSubtreeModified', function(mutationEvent){
                        //here you could get the target element (it's in the mutationEvent) and update the store with it.
                        console.log('dom Modified',mutationEvent);   
                    }, false);
                    this.setDisabled(true); //disabled it to prevent multiple listeners for the same event.
                }
            }
        },
        {
            xtype: 'button',
            text: 'change element',
            handler: function() {
                var domCont= this.up().getComponent('htmlContainer').el.dom;
                domCont.childNodes[0].childNodes[0].innerText = "it's now: " + Ext.Date.format(new Date(), "Y-m-d H:i:s");
            }
        },
        {
            xtype: 'button',
            text: 'clear element',
            handler: function() {
                var domCont= this.up().getComponent('htmlContainer').el.dom;
                domCont.childNodes[0].childNodes[0].innerText = ""; 
            }
        },
        {
            itemId: 'htmlContainer',
            html: '<div>stuff</div>'
        }
    ] 
});