diff --git a/lib/elements/dom-bind.html b/lib/elements/dom-bind.html
index 4b128b86..67bbf92a 100644
--- a/lib/elements/dom-bind.html
+++ b/lib/elements/dom-bind.html
@@ -57,6 +57,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
       constructor() {
         super();
+        if (Polymer.strictTemplatePolicy) {
+          throw new Error(`strictTemplatePolicy: dom-bind not allowed`);
+        }
         this.root = null;
         this.$ = null;
         this.__children = null;
diff --git a/lib/elements/dom-if.html b/lib/elements/dom-if.html
index dc41e243..87b8b627 100644
--- a/lib/elements/dom-if.html
+++ b/lib/elements/dom-if.html
@@ -255,8 +255,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         if (c$ && c$.length) {
           // use first child parent, for case when dom-if may have been detached
           let parent = c$[0].parentNode;
-          for (let i=0, n; (i<c$.length) && (n=c$[i]); i++) {
-            parent.removeChild(n);
+          // Instance children may be disconnected from parents when dom-if
+          // detaches if a tree was innerHTML'ed
+          if (parent) {
+            for (let i=0, n; (i<c$.length) && (n=c$[i]); i++) {
+              parent.removeChild(n);
+            }
           }
         }
         this.__instance = null;
diff --git a/lib/elements/dom-module.html b/lib/elements/dom-module.html
index cb06b25a..e1817876 100644
--- a/lib/elements/dom-module.html
+++ b/lib/elements/dom-module.html
@@ -15,6 +15,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
   let modules = {};
   let lcModules = {};
+  function setModule(id, module) {
+    // store id separate from lowercased id so that
+    // in all cases mixedCase id will stored distinctly
+    // and lowercase version is a fallback
+    modules[id] = lcModules[id.toLowerCase()] = module;
+  }
   function findModule(id) {
     return modules[id] || lcModules[id.toLowerCase()];
   }
@@ -124,12 +130,14 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     register(id) {
       id = id || this.id;
       if (id) {
+        // Under strictTemplatePolicy, reject and null out any re-registered
+        // dom-module since it is ambiguous whether first-in or last-in is trusted 
+        if (Polymer.strictTemplatePolicy && findModule(id) !== undefined) {
+          setModule(id, null);
+          throw new Error(`strictTemplatePolicy: dom-module ${id} re-registered`);
+        }
         this.id = id;
-        // store id separate from lowercased id so that
-        // in all cases mixedCase id will stored distinctly
-        // and lowercase version is a fallback
-        modules[id] = this;
-        lcModules[id.toLowerCase()] = this;
+        setModule(id, this);
         styleOutsideTemplateCheck(this);
       }
     }
diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index 617c856c..e1f43dea 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -153,22 +153,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
           return info.observers;
         }
 
-        /**
-         * @return {HTMLTemplateElement} template for this class
-         */
-        static get template() {
-          // get template first from any imperative set in `info._template`
-          return info._template ||
-            // next look in dom-module associated with this element's is.
-            Polymer.DomModule && Polymer.DomModule.import(this.is, 'template') ||
-            // next look for superclass template (note: use superclass symbol
-            // to ensure correct `this.is`)
-            Base.template ||
-            // finally fall back to `_template` in element's prototype.
-            this.prototype._template ||
-            null;
-        }
-
         /**
          * @return {void}
          */
@@ -371,6 +355,15 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       const klass = GenerateClassFromInfo(info, baseWithMixin);
       // decorate klass with registration info
       klass.is = info.is;
+      // To match 1.x behavior, `_template` supplied on a behavior should take
+      // precedence over dom-module lookup for `is`; this also prevents
+      // dom-module injection under strictTemplatePolicy for an element that
+      // would normally get its template from a behavior.
+      // TODO(sorvell): Remove once "flattened behaviors" lands, since both
+      // `_template` and `is` will be on the same class after that change
+      if (klass.prototype._template !== undefined) {
+        klass.prototype._template = klass.prototype._template;
+      }
       return klass;
     };
 
diff --git a/lib/mixins/element-mixin.html b/lib/mixins/element-mixin.html
index f14c7ffb..b3fd9f99 100644
--- a/lib/mixins/element-mixin.html
+++ b/lib/mixins/element-mixin.html
@@ -130,7 +130,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     /**
-     * Returns a memoized version of the the `observers` array.
+     * Returns a memoized version of the `observers` array.
      * @param {PolymerElementConstructor} constructor Element class
      * @return {Array} Array containing own observers for the given class
      * @protected
@@ -154,7 +154,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * alter these settings. However, additional `observers` may be added
      * by subclasses.
      *
-     * The info object should may contain property metadata as follows:
+     * The info object should contain property metadata as follows:
      *
      * * `type`: {function} type to which an attribute matching the property
      * is deserialized. Note the property is camel-cased from a dash-cased
@@ -166,7 +166,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * property 'foo',
      *
      * * `computed`: {string} creates a computed property. A computed property
-     * also automatically is set to `readOnly: true`. The value is calculated
+     * is also automatically set to `readOnly: true`. The value is calculated
      * by running a method and arguments parsed from the given string. For
      * example 'compute(foo)' will compute a given property when the
      * 'foo' property changes by executing the 'compute' method. This method
@@ -277,6 +277,27 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     /**
+     * Look up template from dom-module for element
+     *
+     * @param {!string} is Element name to look up
+     * @return {!HTMLTemplateElement} Template found in dom module, or
+     *   undefined if not found
+     * @protected
+     */
+    function getTemplateFromDomModule(is) {
+      let template = null;
+      if (is && Polymer.DomModule) {
+        template = Polymer.DomModule.import(is, 'template');
+        // Under strictTemplatePolicy, require any element with an `is`
+        // specified to have a dom-module
+        if (Polymer.strictTemplatePolicy && !template) {
+          throw new Error(`strictTemplatePolicy: expecting dom-module or null template for ${is}`);
+        }
+      }
+      return template;
+    }
+
+  /**
      * @polymer
      * @mixinClass
      * @unrestricted
@@ -374,7 +395,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        *   class MySubClass extends MySuperClass {
        *     static get template() {
        *       if (!memoizedTemplate) {
-       *         memoizedTemplate = super.template.cloneNode(true);
+       *         memoizedTemplate = MySuperClass.template.cloneNode(true);
        *         let subContent = document.createElement('div');
        *         subContent.textContent = 'This came from MySubClass';
        *         memoizedTemplate.content.appendChild(subContent);
@@ -386,17 +407,43 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {HTMLTemplateElement|string} Template to be stamped
        */
       static get template() {
+        // Explanation of template-related properties:
+        // - constructor.template (this getter): the template for the class.
+        //     This can come from the prototype (for legacy elements), from a
+        //     dom-module, or from the super class's template (or can be overridden
+        //     altogether by the user)
+        // - constructor._template: memoized version of constructor.template
+        // - prototype._template: working template for the element, which will be
+        //     parsed and modified in place. It is a cloned version of
+        //     constructor.template, saved in _finalizeClass(). Note that before
+        //     this getter is called, for legacy elements this could be from a
+        //     _template field on the info object passed to Polymer(), a behavior,
+        //     or set in registered(); once the static getter runs, a clone of it
+        //     will overwrite it on the prototype as the working template.
         if (!this.hasOwnProperty(JSCompiler_renameProperty('_template', this))) {
-          this._template = Polymer.DomModule && Polymer.DomModule.import(
-            /** @type {PolymerElementConstructor}*/ (this).is, 'template') ||
-            // note: implemented so a subclass can retrieve the super
-            // template; call the super impl this way so that `this` points
-            // to the superclass.
-            Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.template;
+          this._template =
+            // If user has put template on prototype (e.g. in legacy via registered
+            // callback or info object), prefer that first
+            this.prototype.hasOwnProperty(JSCompiler_renameProperty('_template', this.prototype)) ?
+            this.prototype._template :
+            // Look in dom-module associated with this element's is
+            (getTemplateFromDomModule(/** @type {PolymerElementConstructor}*/ (this).is) ||
+            // Next look for superclass template (call the super impl this
+            // way so that `this` points to the superclass)
+            Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.template);
         }
         return this._template;
       }
 
+      /**
+       * Set the template.
+       *
+       * @param {!HTMLTemplateElement|string} value Template to set.
+       */
+      static set template(value) {
+        this._template = value;
+      }
+
       /**
        * Path matching the url from which the element was imported.
        *
@@ -404,7 +451,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * The `importPath` property is also set on element instances and can be
        * used to create bindings relative to the import path.
        *
-       * For elements defined in ES modules, users should implement 
+       * For elements defined in ES modules, users should implement
        * `static get importMeta() { return import.meta; }`, and the default
        * implementation of `importPath` will  return `import.meta.url`'s path.
        * For elements defined in HTML imports, this getter will return the path
diff --git a/lib/utils/templatize.html b/lib/utils/templatize.html
index f8bd3d64..6aa7711b 100644
--- a/lib/utils/templatize.html
+++ b/lib/utils/templatize.html
@@ -513,6 +513,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @suppress {invalidCasts}
        */
       templatize(template, owner, options) {
+        // Under strictTemplatePolicy, the templatized element must be owned
+        // by a (trusted) Polymer element, indicated by existence of _methodHost;
+        // e.g. for dom-if & dom-repeat in main document, _methodHost is null
+        if (Polymer.strictTemplatePolicy && !findMethodHost(template)) {
+          throw new Error('strictTemplatePolicy: template owner not trusted');
+        }
         options = /** @type {!TemplatizeOptions} */(options || {});
         if (template.__templatizeOwner) {
           throw new Error('A <template> can only be templatized once');
diff --git a/test/runner.html b/test/runner.html
index cde4c190..78b551fa 100644
--- a/test/runner.html
+++ b/test/runner.html
@@ -45,6 +45,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       'unit/custom-style-async.html',
       'unit/custom-style-scope-cache.html',
       'unit/events.html',
+      'unit/strict-template-policy.html',
       'unit/template-whitespace.html',
       'unit/resolveurl.html',
       'unit/case-map.html',
diff --git a/test/unit/behaviors.html b/test/unit/behaviors.html
index 34d94097..98cf74dd 100644
--- a/test/unit/behaviors.html
+++ b/test/unit/behaviors.html
@@ -22,6 +22,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     </template>
   </dom-module>
 
+  <dom-module id="template-from-base">
+    <template>
+      <div id="from-base">should not be used</div>
+    </template>
+  </dom-module>
+
   <script>
   HTMLImports.whenReady(function() {
     window.BehaviorA = {
@@ -283,6 +289,57 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
       is: 'behavior-registered'
     });
+
+    window.templateBehavior1 = {
+      _template: Polymer.html`<div id="from-behavior1"></div>`
+    };
+
+    window.templateBehavior2 = {
+      _template: Polymer.html`<div id="from-behavior2"></div>`
+    };
+
+    window.templateBehaviorFromRegistered = {
+      registered: function() {
+        this._template = Polymer.html`<div id="behavior-from-registered"></div>`;
+      }
+    };
+
+    Polymer({
+      is: 'template-from-registered',
+      registered: function() {
+        this._template = Polymer.html`<div id="from-registered"></div>`;
+      }
+    });
+
+    Polymer({
+      is: 'template-from-base',
+      behaviors: [
+        window.templateBehavior1
+      ]
+    });
+
+    Polymer({
+      is: 'template-from-behavior',
+      behaviors: [
+        window.templateBehavior1
+      ]
+    });
+
+    Polymer({
+      is: 'template-from-behavior-overridden',
+      behaviors: [
+        window.templateBehavior1,
+        window.templateBehavior2
+      ]
+    });
+
+    Polymer({
+      is: 'template-from-behavior-registered',
+      behaviors: [
+        window.templateBehaviorFromRegistered
+      ]
+    });
+
   });
   </script>
 
@@ -309,6 +366,37 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       <behavior-registered></behavior-registered>
     </template>
   </test-fixture>
+
+  <test-fixture id="from-registered">
+    <template>
+      <template-from-registered></template-from-registered>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="from-base">
+    <template>
+      <template-from-base></template-from-base>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="from-behavior">
+    <template>
+      <template-from-behavior></template-from-behavior>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="from-behavior-overridden">
+    <template>
+      <template-from-behavior-overridden></template-from-behavior-overridden>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="from-behavior-registered">
+    <template>
+      <template-from-behavior-registered></template-from-behavior-registered>
+    </template>
+  </test-fixture>
+  
 <script>
 
 suite('single behavior element', function() {
@@ -505,6 +593,37 @@ suite('nested-behaviors element', function() {
 
 });
 
+suite('templates from behaviors', function() {
+
+  test('template from registered callback', function() {
+    var el = fixture('from-registered');
+    assert.ok(el.shadowRoot.querySelector('#from-registered'));
+  });
+
+  test('template from base', function() {
+    var el = fixture('from-base');
+    assert.notOk(el.shadowRoot.querySelector('#from-base'));
+    assert.ok(el.shadowRoot.querySelector('#from-behavior1'));
+  });
+
+  test('template from behavior', function() {
+    var el = fixture('from-behavior');
+    assert.ok(el.shadowRoot.querySelector('#from-behavior1'));
+  });
+
+  test('template from overriding behavior', function() {
+    var el = fixture('from-behavior-overridden');
+    assert.notOk(el.shadowRoot.querySelector('#from-behavior1'));
+    assert.ok(el.shadowRoot.querySelector('#from-behavior2'));
+  });
+
+  test('template from behavior registered callback', function() {
+    var el = fixture('from-behavior-registered');
+    assert.ok(el.shadowRoot.querySelector('#behavior-from-registered'));
+  });
+
+});
+
 </script>
 
 </body>
diff --git a/test/unit/strict-template-policy.html b/test/unit/strict-template-policy.html
new file mode 100644
index 00000000..8596bda4
--- /dev/null
+++ b/test/unit/strict-template-policy.html
@@ -0,0 +1,359 @@
+<!doctype html>
+<!--
+@license
+Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+-->
+<html>
+<head>
+  <script src="../../../webcomponentsjs/webcomponents-lite.js"></script>
+  <script>
+    Polymer = {
+      strictTemplatePolicy: true
+    };
+    // Errors thrown in custom element reactions are not thrown up
+    // the call stack to the dom methods that provoked them, so need
+    // to catch them here and prevent mocha from complaining about them
+    window.addEventListener('error', event => {
+      if (window.uncaughtErrorFilter && window.uncaughtErrorFilter(event)) {
+        event.preventDefault();
+        event.stopImmediatePropagation();
+      }
+    });
+  </script>
+  <script src="../../../web-component-tester/browser.js"></script>
+  <link rel="import" href="../../polymer.html">
+  <script>
+    HTMLImports.whenReady(() => {
+      // Errors thrown in Polymer's debouncer queue get re-thrown
+      // via setTimeout, making them particulary difficult to verify;
+      // Wrap debouncer callbacks and store on the globalError to test later
+      const debounce = Polymer.Debouncer.debounce;
+      Polymer.Debouncer.debounce = function(debouncer, asyncModule, callback) {
+        return debounce(debouncer, asyncModule, function() {
+          try { 
+            callback(); 
+          } catch(error) {
+            if (!window.uncaughtErrorFilter || !window.uncaughtErrorFilter(error)) {
+              throw error;
+            }
+          }
+        });
+      };
+    });
+  </script>
+</head>
+<body>
+
+  <dom-module id="trusted-element">
+    <template>Trusted</template>
+    <script>
+      HTMLImports.whenReady(function() {
+        class TrustedElement extends Polymer.Element {
+          static get is() { return 'trusted-element'; }
+        }
+        customElements.define(TrustedElement.is, TrustedElement);
+      });
+    </script>
+  </dom-module>
+    
+  <dom-module id="trusted-element-legacy">
+    <template>Trusted</template>
+    <script>
+      HTMLImports.whenReady(function() {
+        Polymer({is: 'trusted-element-legacy'});
+      });
+    </script>
+  </dom-module>
+
+  <dom-module id="trusted-templates">
+    <template>
+      <dom-repeat items="[0]">
+        <template>
+          <div id="dom-repeat-ok"></div>
+          <dom-if if>
+            <template><div id="nested-dom-if-ok"></div></template>
+          </dom-if>
+        </template>
+      </dom-repeat>
+      <dom-if if>
+        <template>
+          <div id="dom-if-ok"></div>
+          <dom-repeat items="[0]">
+            <template>
+              <div id="nested-dom-repeat-ok"></div>
+            </template>
+          </dom-repeat>
+        </template>
+      </dom-if>
+    </template>
+    <script>
+      HTMLImports.whenReady(function() {
+        class TrustedTemplates extends Polymer.Element {
+          static get is() { return 'trusted-templates'; }
+        }
+        customElements.define(TrustedTemplates.is, TrustedTemplates);
+      });
+    </script>
+  </dom-module>
+
+  <dom-module id="trusted-templates-legacy">
+    <template>
+      <template is="dom-repeat" items="[0]">
+        <div id="dom-repeat-ok"></div>
+        <template is="dom-if" if><div id="nested-dom-if-ok"></div></template>
+      </template>
+      <template is="dom-if" if>
+        <div id="dom-if-ok"></div>
+        <template is="dom-repeat" items="[0]"><div id="nested-dom-repeat-ok"></div></template>
+      </template>
+    </template>
+    <script>
+      HTMLImports.whenReady(function() {
+        Polymer({is: 'trusted-templates-legacy'});
+      });
+    </script>
+  </dom-module>
+
+  <div id="target"></div>
+
+  <script>
+    suite('strictTemplatePolicy', function() {
+
+      // Capture mocha's default onerror
+      const onerror = window.onerror;
+      const topOnerror = window.top.topOnerror;
+
+      function restoreOnError() {
+        window.onerror = onerror;
+        window.top.onerror = topOnerror;
+        window.uncaughtErrorFilter = window.top.uncaughtErrorFilter = null;
+      }
+
+      teardown(function() {
+        // Restore mocha's onerror
+        restoreOnError();
+        document.getElementById('target').textContent = '';
+      });
+
+      // Errors thrown in custom element reactions are not thrown up
+      // the call stack to the dom methods that provoked them, so this
+      // wraps Chai's assert.throws to re-throw uncaught errors
+      function assertThrows(fn, re) {
+        // Safari does not forward error messages
+        re = new RegExp('(' + re.toString().slice(1,-1) + ')|(Script error\\.)', re.flags);
+        // Catch uncaught errors; note when running in iframe sometimes
+        // Safari errors are thrown on the top window, sometimes not, so
+        // catch in both places
+        let uncaughtError = null;
+        window.onerror = window.top.onerror = window.uncaughtErrorFilter = 
+          window.top.uncaughtErrorFilter = function(err) {
+          if (!uncaughtError) {
+            uncaughtError = err instanceof Error ? err : new Error(err.message || err);
+          }
+          return true;
+        };
+        assert.throws(function() {
+          fn();
+          // Re-throw any uncaughtErrors
+          if (uncaughtError) {
+            throw uncaughtError;
+          }
+          // Force polyfill reactions and/or async template stamping
+          Polymer.flush();
+          // Re-throw any uncaughtErrors
+          if (uncaughtError) {
+            throw uncaughtError;
+          }
+        }, re);
+        restoreOnError();
+      }
+
+      test('dom-bind', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+          '<dom-bind>' +
+          '  <template>' +
+          '    <div id="injected"></div>'+
+          '  </template>`' + 
+          '</dom-bind>';
+        }, /dom-bind not allowed/);
+        assert.notOk(document.getElementById('injected'));
+      });
+
+      test('dom-if', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-if if>' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>' + 
+            '</dom-if>';
+        }, /template owner not trusted/);
+        assert.notOk(document.getElementById('injected'));
+      });
+
+      test('dom-repeat', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-repeat items="[0]">' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>`' + 
+            '</dom-repeat>';
+        }, /template owner not trusted/);
+        assert.notOk(document.getElementById('injected'));
+      });
+
+      test('dom-module after registration', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-module id="trusted-element">' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>`' +
+            '</dom-module>';
+        }, /trusted-element re-registered/);
+        let el;
+        assertThrows(function() {
+          el = document.createElement('trusted-element');
+          document.getElementById('target').appendChild(el);
+        }, /expecting dom-module or null template for trusted-element/);
+        assert.notOk(el && el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+      
+      test('dom-module after registration, again', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-module id="trusted-element">' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>`' +
+            '</dom-module>';
+        }, /trusted-element re-registered/);
+        const el = document.createElement('trusted-element');
+        document.getElementById('target').appendChild(el);
+        assert.notOk(el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+      
+      test('dom-module before registration', function() {
+        document.getElementById('target').innerHTML =
+          '<dom-module id="has-no-template">' +
+          '  <template>' +
+          '    <div id="injected"></div>'+
+          '  </template>`' +
+          '</dom-module>';
+        class HasNoTemplate extends Polymer.Element {
+          static get is() { return 'has-no-template'; }
+          static get template() { return null; }
+        }
+        customElements.define(HasNoTemplate.is, HasNoTemplate);
+        let el = document.createElement('has-no-template');
+        document.getElementById('target').appendChild(el);
+        assert.notOk(el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+
+      test('dom-module after registration (legacy)', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-module id="trusted-element-legacy">' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>`' +
+            '</dom-module>';
+        }, /trusted-element-legacy re-registered/);
+        let el;
+        assertThrows(function() {
+          el = document.createElement('trusted-element-legacy');
+          document.getElementById('target').appendChild(el);
+        }, /expecting dom-module or null template for trusted-element-legacy/);
+        assert.notOk(el && el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+      
+      test('dom-module after registration, again (legacy)', function() {
+        assertThrows(function() {
+          document.getElementById('target').innerHTML =
+            '<dom-module id="trusted-element-legacy">' +
+            '  <template>' +
+            '    <div id="injected"></div>'+
+            '  </template>`' +
+            '</dom-module>';
+        }, /trusted-element-legacy re-registered/);
+        const el = document.createElement('trusted-element-legacy');
+        document.getElementById('target').appendChild(el);
+        assert.notOk(el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+      
+      test('dom-module before registration (legacy)', function() {
+        document.getElementById('target').innerHTML =
+          '<dom-module id="has-no-template-legacy">' +
+          '  <template>' +
+          '    <div id="injected"></div>'+
+          '  </template>`' +
+          '</dom-module>';
+        Polymer({
+          is: 'has-no-template-legacy',
+          _template: null
+        });
+        let el = document.createElement('has-no-template-legacy');
+        document.getElementById('target').appendChild(el);
+        assert.notOk(el.shadowRoot);
+        assert.notOk(document.getElementById('injected'));
+      });
+
+      test('element without explicit template throws', function() {
+        assertThrows(function() {
+          class HasNoTemplateThrows extends Polymer.Element {
+            static get is() { return 'has-no-template-throws'; }
+          }
+          customElements.define(HasNoTemplateThrows.is, HasNoTemplateThrows); 
+          var el = document.createElement('has-no-template-throws');
+          document.getElementById('target').appendChild(el);
+        }, /expecting dom-module or null template/);
+      });
+
+      test('element without explicit template throws (legacy)', function() {
+        assertThrows(function() {
+          Polymer({
+            is: 'has-no-template-throws-legacy'
+          });
+          var el = document.createElement('has-no-template-throws-legacy');
+          document.getElementById('target').appendChild(el);
+        }, /expecting dom-module or null template/);
+      });
+
+      test('template helpers in trusted templates work', function() {        
+        var el = document.createElement('trusted-templates');
+        document.getElementById('target').appendChild(el);
+        Polymer.flush();
+        assert.ok(el.shadowRoot.querySelector('#dom-repeat-ok'));
+        assert.ok(el.shadowRoot.querySelector('#dom-if-ok'));
+        assert.ok(el.shadowRoot.querySelector('#nested-dom-repeat-ok'));
+        assert.ok(el.shadowRoot.querySelector('#nested-dom-if-ok'));
+      });
+
+      test('template helpers in trusted templates work (legacy)', function() {
+        var el = document.createElement('trusted-templates-legacy');
+        document.getElementById('target').appendChild(el);
+        Polymer.flush();
+        assert.ok(el.shadowRoot.querySelector('#dom-repeat-ok'));
+        assert.ok(el.shadowRoot.querySelector('#dom-if-ok'));
+        assert.ok(el.shadowRoot.querySelector('#nested-dom-repeat-ok'));
+        assert.ok(el.shadowRoot.querySelector('#nested-dom-if-ok'));
+      });
+
+    });
+
+  </script>
+    
+</body>
+</html>
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