The source code

    /**
     * @class Ext.form.Label
     * @extends Ext.BoxComponent
     * Basic Label field.
     * @constructor
     * Creates a new Label
     * @param {Ext.Element/String/Object} config The configuration options.  If an element is passed, it is set as the internal
     * element and its id used as the component id.  If a string is passed, it is assumed to be the id of an existing element
     * and is used as the component id.  Otherwise, it is assumed to be a standard config object and is applied to the component.
     * @xtype label
     */
    Ext.form.Label = Ext.extend(Ext.BoxComponent, {
        /**
         * @cfg {String} text The plain text to display within the label (defaults to ''). If you need to include HTML
         * tags within the label's innerHTML, use the {@link #html} config instead.
         */
        /**
         * @cfg {String} forId The id of the input element to which this label will be bound via the standard HTML 'for'
         * attribute. If not specified, the attribute will not be added to the label.
         */
        /**
         * @cfg {String} html An HTML fragment that will be used as the label's innerHTML (defaults to '').
         * Note that if {@link #text} is specified it will take precedence and this value will be ignored.
         */

        // private
        onRender : function(ct, position){
            if(!this.el){
                this.el = document.createElement('label');
                this.el.id = this.getId();
                this.el.innerHTML = this.text ? Ext.util.Format.htmlEncode(this.text) : (this.html || '');
                if(this.forId){
                    this.el.setAttribute('for', this.forId);
                }
            }
            Ext.form.Label.superclass.onRender.call(this, ct, position);
        },

        /**
         * Updates the label's innerHTML with the specified string.
         * @param {String} text The new label text
         * @param {Boolean} encode (optional) False to skip HTML-encoding the text when rendering it
         * to the label (defaults to true which encodes the value). This might be useful if you want to include
         * tags in the label's innerHTML rather than rendering them as string literals per the default logic.
         * @return {Label} this
         */
        setText : function(t, encode){
            var e = encode === false;
            this[!e ? 'text' : 'html'] = t;
            delete this[e ? 'text' : 'html'];
            if(this.rendered){
                this.el.dom.innerHTML = encode !== false ? Ext.util.Format.htmlEncode(t) : t;
            }
            return this;
        }
    });

    Ext.reg('label', Ext.form.Label);