How do I display a YouTube video on a Panel in ExtJS?

Take a look at the html property of the Panel, that you can use XTemplate, which you can find in this link an example using html.

Panel:

                xtype: 'panel',
                border: false,
                autoScroll: true,

                id:'seupanel',
                html: '\n\
                <table border="3" >\n\
                <tr>\n\
                </tr>
                </table>\n\
                ',

Your Xtemplate:

                var tpl = new Ext.XTemplate('
                    <seu html aqui>
                );
                tpl.overwrite(panel.body, data);

I used to display pure html within a panel;

Take a look at this extension:

  

EXTJS and YouTube Integration

     

Pohon.ux.Youtube is package of ExtJS 3.x components which enables integration of youtube videos with your ExtJS application

By default, EXTJS does not have this functionality, but has this extension that solves the issue well.