JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://extjs.cachefly.net/ext-4.0.2/resources/css/ext-all.css">

JavaScript

Ext.onReady(function() { // Every property is declared inside the class
    Ext.define('MyCustomPanel1', {
        extend: 'Ext.panel.Panel',
        alias: 'mycustompanel1',
        title: 'I am a custom Panel 1',
        html: 'This is the content of my custom panel 1',
        renderTo: Ext.getBody()
    });    
    
    
    Ext.define('MyCustomPanel2', { // HTML is declared inside the class, title inside the config, constructor overridden
        extend: 'Ext.panel.Panel',
        alias: 'mycustompanel2',
        renderTo: Ext.getBody(),        
        html: 'This is the content of my custom panel 2',        
        config: {
            title: 'I am a custom Panel 2'
        },
        constructor: function(config) {
            this.callParent(arguments);
            this.initConfig(config)
        }
    });        
    
    
    Ext.define('MyCustomPanel3', { // Title and HTML declared inside config, constructor overridden
        extend: 'Ext.panel.Panel',
        alias: 'mycustompanel3',
        renderTo: Ext.getBody(),        
        config: {
            title: 'I am a custom Panel 3',
            html: 'This is the content of my custom panel 3'
        },
        constructor: function(config) {
            this.callParent(arguments);
            this.initConfig(config)
        }
    });
    
    Ext.define('MyCustomPanel4', { // title and html inside of initComponent, title override in instance declaration doesn't hold. HTML property is empty on render
        extend: 'Ext.panel.Panel',
        alias: 'mycustompanel4',
        renderTo: Ext.getBody(),        
        initComponent: function(config) {
            Ext.apply(this, {
                title: 'I am a custom Panel 4',
                html: 'This is the content of my custom panel 4'                
            })
            this.callParent(arguments);
        }
    });            
    Ext.define('MyCustomPanel5', { // title declared inside config, html set inside of initComponent. Both...