Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 32672)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -157,18 +157,40 @@
 		defaultActiveArguments: { duration: 'fast', completeCallback: $.noop },
 		defaultExpandedArguments: { duration: 'fast', completeCallback: $.noop },
 		containerType: 'container',
+		defaults: {
+			title: '',
+			description: '',
+			priority: 100,
+			type: 'default',
+			content: null,
+			active: true,
+			instanceNumber: null
+		},
 
 		/**
 		 * @since 4.1.0
 		 *
-		 * @param {String} id
-		 * @param {Object} options
+		 * @param {string}         id - The ID for the container.
+		 * @param {object}         options - Object containing one property: params.
+		 * @param {object}         options.params - Object containing the following properties.
+		 * @param {string}         options.params.title - Title shown when panel is collapsed and expanded.
+		 * @param {string=}        [options.params.description] - Description shown at the top of the panel.
+		 * @param {number=100}     [options.params.priority] - The sort priority for the panel.
+		 * @param {string=default} [options.params.type] - The type of the panel. See wp.customize.panelConstructor.
+		 * @param {string=}        [options.params.content] - The markup to be used for the panel container. If empty, a JS template is used.
+		 * @param {boolean=true}   [options.params.active] - Whether the panel is active or not.
 		 */
 		initialize: function ( id, options ) {
 			var container = this;
 			container.id = id;
-			container.params = {};
-			$.extend( container, options || {} );
+			options = options || {};
+
+			options.params = _.defaults(
+				options.params || {},
+				container.defaults
+			);
+
+			$.extend( container, options );
 			container.templateSelector = 'customize-' + container.containerType + '-' + container.params.type;
 			container.container = $( container.params.content );
 			if ( 0 === container.container.length ) {
@@ -202,7 +224,7 @@
 
 			api.utils.bubbleChildValueChanges( container, [ 'priority', 'active' ] );
 
-			container.priority.set( isNaN( container.params.priority ) ? 100 : container.params.priority );
+			container.priority.set( container.params.priority );
 			container.active.set( container.params.active );
 			container.expanded.set( false );
 		},
@@ -386,10 +408,12 @@
 
 			if ( 0 !== $( '#tmpl-' + container.templateSelector ).length ) {
 				template = wp.template( container.templateSelector );
-				if ( template && container.container ) {
-					return $.trim( template( container.params ) );
-				}
+			} else {
+				template = wp.template( 'customize-' + container.containerType + '-default' );
 			}
+			if ( template && container.container ) {
+				return $.trim( template( container.params ) );
+			}
 
 			return '<li></li>';
 		}
@@ -403,12 +427,32 @@
 	 */
 	api.Section = Container.extend({
 		containerType: 'section',
+		defaults: {
+			title: '',
+			description: '',
+			priority: 100,
+			type: 'default',
+			content: null,
+			active: true,
+			instanceNumber: null,
+			panel: null,
+			customizeAction: ''
+		},
 
 		/**
 		 * @since 4.1.0
 		 *
-		 * @param {String} id
-		 * @param {Array}  options
+		 * @param {string}         id - The ID for the section.
+		 * @param {object}         options - Object containing one property: params.
+		 * @param {object}         options.params - Object containing the following properties.
+		 * @param {string}         options.params.title - Title shown when section is collapsed and expanded.
+		 * @param {string=}        [options.params.description] - Description shown at the top of the section.
+		 * @param {number=100}     [options.params.priority] - The sort priority for the section.
+		 * @param {string=default} [options.params.type] - The type of the section. See wp.customize.sectionConstructor.
+		 * @param {string=}        [options.params.content] - The markup to be used for the section container. If empty, a JS template is used.
+		 * @param {boolean=true}   [options.params.active] - Whether the section is active or not.
+		 * @param {string}         options.params.panel - The ID for the panel this section is associated with.
+		 * @param {string=}        [options.params.customizeAction] - Additional context information shown before the section title when expanded.
 		 */
 		initialize: function ( id, options ) {
 			var section = this;
@@ -1009,8 +1053,15 @@
 		/**
 		 * @since 4.1.0
 		 *
-		 * @param  {String} id
-		 * @param  {Object} options
+		 * @param {string}         id - The ID for the panel.
+		 * @param {object}         options - Object containing one property: params.
+		 * @param {object}         options.params - Object containing the following properties.
+		 * @param {string}         options.params.title - Title shown when panel is collapsed and expanded.
+		 * @param {string=}        [options.params.description] - Description shown at the top of the panel.
+		 * @param {number=100}     [options.params.priority] - The sort priority for the panel.
+		 * @param {string=default} [options.params.type] - The type of the panel. See wp.customize.panelConstructor.
+		 * @param {string=}        [options.params.content] - The markup to be used for the panel container. If empty, a JS template is used.
+		 * @param {boolean=true}   [options.params.active] - Whether the panel is active or not.
 		 */
 		initialize: function ( id, options ) {
 			var panel = this;
@@ -1216,10 +1267,12 @@
 			// Add the content to the container.
 			if ( 0 !== $( '#tmpl-' + panel.templateSelector + '-content' ).length ) {
 				template = wp.template( panel.templateSelector + '-content' );
-				if ( template && panel.container ) {
-					panel.container.find( '.accordion-sub-container' ).html( template( panel.params ) );
-				}
+			} else {
+				template = wp.template( 'customize-panel-default-content' );
 			}
+			if ( template && panel.container ) {
+				panel.container.find( '.accordion-sub-container' ).html( template( panel.params ) );
+			}
 		}
 	});
 
Index: tests/qunit/fixtures/customize-settings.js
===================================================================
--- tests/qunit/fixtures/customize-settings.js	(revision 32672)
+++ tests/qunit/fixtures/customize-settings.js	(working copy)
@@ -57,7 +57,16 @@
 			'priority': 110,
 			'title': 'Fixture titleless panel using template',
 			'type': 'titleless'
-		}
+		},
+		'fixture-panel-reusing-default-template': {
+			'active': true,
+			'description': 'Lorem ipsum',
+			'instanceNumber': 3,
+			'priority': 110,
+			'title': 'Fixture panel of custom type re-using default template',
+			'type': 'reusing-default-template'
+		},
+		'fixture-panel-without-params': {}
 	},
 	'sections': {
 		'fixture-section': {
@@ -87,7 +96,17 @@
 			'priority': 20,
 			'title': 'Fixture titleless section using template',
 			'type': 'titleless'
-		}
+		},
+		'fixture-section-reusing-default-template': {
+			'active': true,
+			'description': '',
+			'instanceNumber': 4,
+			'panel': 'fixture-panel',
+			'priority': 20,
+			'title': 'Fixture section of custom type re-using default template',
+			'type': 'reusing-default-template'
+		},
+		'fixture-section-without-params': {}
 	},
 	'settings': {
 		'fixture-setting': {
Index: tests/qunit/wp-admin/js/customize-controls.js
===================================================================
--- tests/qunit/wp-admin/js/customize-controls.js	(revision 32672)
+++ tests/qunit/wp-admin/js/customize-controls.js	(working copy)
@@ -136,7 +136,38 @@
 		ok( 0 === section.container.find( '> .accordion-section-title' ).length );
 		ok( 1 === section.container.find( '> .accordion-section-content' ).length );
 	} );
+	module( 'Customizer Custom Type Section Lacking Specific Template' );
+	test( 'Fixture section has expected content', function () {
+		var id = 'fixture-section-reusing-default-template', section;
+		section = wp.customize.section( id );
+		ok( ! section.params.content );
+		ok( !! section.container );
+		ok( section.container.is( '.control-section.control-section-' + section.params.type ) );
+		ok( 1 === section.container.find( '> .accordion-section-title' ).length );
+		ok( 1 === section.container.find( '> .accordion-section-content' ).length );
+	} );
+	module( 'Customizer Section lacking any params' );
+	test( 'Fixture section has default params supplied', function () {
+		var id = 'fixture-section-without-params', section, defaultParams;
+		section = wp.customize.section( id );
+		defaultParams = {
+			title: '',
+			description: '',
+			priority: 100,
+			panel: null,
+			type: 'default',
+			content: null,
+			active: true,
+			instanceNumber: null,
+			customizeAction: ''
+		};
+		jQuery.each( defaultParams, function ( key, value ) {
+			ok( 'undefined' !== typeof section.params[ key ] );
+			equal( value, section.params[ key ] );
+		} );
+	} );
 
+
 	// Begin panels.
 	module( 'Customizer Panel in Fixture' );
 	test( 'Fixture panel exists', function () {
@@ -199,6 +230,34 @@
 		ok( 1 === panel.container.find( '> .control-panel-content' ).length );
 	} );
 
+	module( 'Customizer Custom Type Panel Lacking Specific Template' );
+	test( 'Fixture panel has expected content', function () {
+		var id = 'fixture-panel-reusing-default-template', panel;
+		panel = wp.customize.panel( id );
+		ok( ! panel.params.content );
+		ok( !! panel.container );
+		ok( panel.container.is( '.control-panel.control-panel-' + panel.params.type ) );
+		ok( 1 === panel.container.find( '> .accordion-section-title' ).length );
+		ok( 1 === panel.container.find( '> .control-panel-content' ).length );
+	} );
+	module( 'Customizer Panel lacking any params' );
+	test( 'Fixture panel has default params supplied', function () {
+		var id = 'fixture-panel-without-params', panel, defaultParams;
+		panel = wp.customize.panel( id );
+		defaultParams = {
+			title: '',
+			description: '',
+			priority: 100,
+			type: 'default',
+			content: null,
+			active: true,
+			instanceNumber: null
+		};
+		jQuery.each( defaultParams, function ( key, value ) {
+			ok( 'undefined' !== typeof panel.params[ key ] );
+			equal( value, panel.params[ key ] );
+		} );
+	} );
 
 	module( 'Dynamically-created Customizer Setting Model' );
 	settingId = 'new_blogname';