diff --git a/README.md b/README.md
index ae20ca36e..4059bee0f 100644
--- a/README.md
+++ b/README.md
@@ -29,6 +29,7 @@ A collection of AMP tools making it easier to publish and host AMP pages. The fo
 - **[amp-runtime-version](./packages/runtime-version):** a javascript library for querying the current AMP runtime version.
 - **[amp-update-cache](./packages/update-cache):** a javascript library for updating AMP documents in AMP Caches.
 - **[amp-update-linter](./packages/linter):** a javascript library for linting AMP documents (includes CLI mode).
+- **[amp-validator-rules](./packages/validator-rules):** a javascript library for querying AMP validator rules.
 
 ## Development
 
@@ -43,7 +44,7 @@ git clone https://github.com/your-fork/amp-toolbox.git
 # step into local repo
 cd amp-toolbox
 
-# install dependencies 
+# install dependencies
 npm install
 
 # run tests
diff --git a/package-lock.json b/package-lock.json
index 7f4b487c7..6e24bb66e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -4418,6 +4418,22 @@
         }
       }
     },
+    "@ampproject/toolbox-validator-rules": {
+      "version": "file:packages/validator-rules",
+      "requires": {
+        "@ampproject/toolbox-core": "^1.0.0-beta.3"
+      },
+      "dependencies": {
+        "@ampproject/toolbox-core": {
+          "version": "1.0.0-beta.3",
+          "resolved": "https://registry.npmjs.org/@ampproject/toolbox-core/-/toolbox-core-1.0.0-beta.3.tgz",
+          "integrity": "sha512-IjZQRIRyAQDKA56PXsph2uyXvKQl+cVh+Xz1UX0Am0vIdbwXacHbx/oxYC7O0HD5Gbg6K508tHBeUTJFiUO0iA==",
+          "requires": {
+            "node-fetch": "2.6.0"
+          }
+        }
+      }
+    },
     "@babel/code-frame": {
       "version": "7.0.0",
       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.0.0.tgz",
diff --git a/package.json b/package.json
index 5c8eb39f5..bbc6e06f4 100644
--- a/package.json
+++ b/package.json
@@ -74,6 +74,7 @@
     "@ampproject/toolbox-optimizer": "file:packages/optimizer",
     "@ampproject/toolbox-optimizer-express": "file:packages/optimizer-express",
     "@ampproject/toolbox-runtime-version": "file:packages/runtime-version",
-    "@ampproject/toolbox-update-cache": "file:packages/update-cache"
+    "@ampproject/toolbox-update-cache": "file:packages/update-cache",
+    "@ampproject/toolbox-validator-rules": "file:packages/validator-rules"
   }
 }
diff --git a/packages/validator-rules/.gitignore b/packages/validator-rules/.gitignore
new file mode 100644
index 000000000..6b37003e2
--- /dev/null
+++ b/packages/validator-rules/.gitignore
@@ -0,0 +1 @@
+validator.json
diff --git a/packages/validator-rules/README.md b/packages/validator-rules/README.md
new file mode 100644
index 000000000..caf4db5b5
--- /dev/null
+++ b/packages/validator-rules/README.md
@@ -0,0 +1,89 @@
+# AMP-Toolbox Validator Rules
+
+Queries published AMP Validator rules and extracts information about required
+markup and attributes for all AMP formats.
+
+## Usage
+
+Install via:
+
+```
+$ npm install @ampproject/toolbox-validator-rules@canary
+```
+
+### Including the Module
+
+#### ES Module (Browser)
+
+```javascript
+import validatorRules from '@ampproject/toolbox-validator-rules';
+```
+
+#### CommonJs (Node)
+
+```javascript
+const validatorRules = require('@ampproject/toolbox-validator-rules');
+```
+
+### Using the module
+
+```javascript
+  // Loads the validator rules remotely with default options
+  const rules = await validatorRules.fetch();
+
+
+  // The raw unprocessed rules
+  console.log(rules.raw);
+
+  // All tags, combined with their respective attribute lists
+  console.log(rules.tags);
+
+  // All extensions
+  console.log(rules.extensions);
+
+  // Get all tag names used in AMP for Email
+  // The supported formats are AMP, AMP4EMAIL, AMP4ADS and ACTIONS
+  const tags = rules.getTagsForFormat('AMP4EMAIL');
+
+  // Display their names
+  console.log(tags.map(tag => tag.tagName));
+
+  // Get information about an extension
+  const ext = rules.getExtension('AMP4EMAIL', 'amp-carousel');
+
+  // Display supported versions
+  console.log(ext.versions);
+```
+
+### Format of rules
+
+The rules used closely follow the proto definitions from [validator.proto](https://github.com/ampproject/amphtml/blob/master/validator/validator.proto).
+
+Specifically:
+
+-   The `raw` property is unprocessed [ValidatorRules](https://github.com/ampproject/amphtml/blob/master/validator/validator.proto#L643), the same format used by `https://cdn.ampproject.org/v0/validator.json`
+-   The result of `getTagsForFormat` and the `tags` property is a list of [TagSpec](https://github.com/ampproject/amphtml/blob/b892d81467594cab5473c803e071af5108f834a6/validator/validator.proto#L463)
+-   The result of `getExtension` is [ExtensionSpec](https://github.com/ampproject/amphtml/blob/b892d81467594cab5473c803e071af5108f834a6/validator/validator.proto#L388) with the `htmlFormat` field from `TagSpec`
+-   The `extensions` property a list of [ExtensionSpec](https://github.com/ampproject/amphtml/blob/b892d81467594cab5473c803e071af5108f834a6/validator/validator.proto#L388) with the `htmlFormat` field from `TagSpec`
+-   The `errors` property combines [ErrorFormat](https://github.com/ampproject/amphtml/blob/b892d81467594cab5473c803e071af5108f834a6/validator/validator.proto#L874) and [ErrorSpecificity](https://github.com/ampproject/amphtml/blob/b892d81467594cab5473c803e071af5108f834a6/validator/validator.proto#L869)
+
+### Options
+
+`fetch` optionally accepts an options object allowing you to customize its
+behaviour.
+
+The following options are supported:
+
+   * `noCache`: true to always fetch latest rules (by default, subsequent calls to `fetch` reuse the same result).
+   * `rules`: object to use locally specified rules instead of fetching them from the AMP CDN.
+   * `url`: override the URL where validator rules are fetched from.
+   * `source`: one of `'local'` (load rules from local file named "validator.json"), `'remote'` (fetch rules from CDN) or `'auto'` which is the default (tries looking for the local file first, then tries to fetch from CDN).
+
+Example:
+
+```
+validatorRules.fetch({
+  noCache: true,
+  source: 'remote'
+});
+```
diff --git a/packages/validator-rules/index.js b/packages/validator-rules/index.js
new file mode 100644
index 000000000..0b7812b1d
--- /dev/null
+++ b/packages/validator-rules/index.js
@@ -0,0 +1,39 @@
+/**
+ * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+const loadRules = require('./lib/loadRules');
+const AmpValidatorRules = require('./lib/AmpValidatorRules');
+
+let cached = null;
+
+async function fetch(opt = {}) {
+  if (!opt.noCache && cached) {
+    return cached;
+  }
+
+  let rules = opt.rules;
+
+  if (!rules) {
+    rules = await loadRules(opt);
+  }
+
+  cached = new AmpValidatorRules(rules);
+  return cached;
+}
+
+module.exports = {fetch};
diff --git a/packages/validator-rules/lib/AmpValidatorRules.js b/packages/validator-rules/lib/AmpValidatorRules.js
new file mode 100644
index 000000000..998645418
--- /dev/null
+++ b/packages/validator-rules/lib/AmpValidatorRules.js
@@ -0,0 +1,201 @@
+/**
+ * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class AmpValidatorRules {
+  /**
+   * Creates an instance of AmpValidatorRules.
+   * @param {Object} rules - rules imported from validator.json
+   */
+  constructor(rules) {
+    /**
+     * Unprocessed validator rules.
+     * @type {Object}
+     */
+    this.raw = rules;
+    /**
+     * List of all the tags processed from rules.
+     * @type {Array<Object>}
+     */
+    this.tags = [];
+    /**
+     * List of all the extensions processed from rules.
+     * @type {Array<Object>}
+     */
+    this.extensions = [];
+    /**
+     * Map of errors and their associated format and specificity.
+     * @type {Object}
+     */
+    this.errors = {};
+
+    this.extensionCache_ = {};
+    this.initRules_(rules);
+  }
+
+  /**
+   * Returns the list of supported tags for the given format.
+   *
+   * @param {string} format - Format to return tags for
+   * @param {boolean} [transformed] - Use transformed version of the format
+   * @return {Array<Object>} List of tags supported by the given format
+   */
+  getTagsForFormat(format, transformed = false) {
+    format = format.toLowerCase();
+    return this.tags
+        .filter(
+            (tag) =>
+              tag.htmlFormat.includes(format.toUpperCase()) &&
+          this.checkEntityFormat_(tag, format) &&
+          this.checkEntityTransformed_(tag, transformed)
+        )
+        .map((tag) => {
+          tag = Object.assign({}, tag);
+          tag.attrs = tag.attrs.filter(
+              (attr) =>
+                this.checkEntityFormat_(attr, format) &&
+            this.checkEntityTransformed_(attr, transformed)
+          );
+          return tag;
+        });
+  }
+
+  /**
+   * Returns the AMP extension spec for the given format and name.
+   *
+   * @param {string} format - Format to filter on
+   * @param {string} extension - Extension name
+   * @return {Object} Extension spec
+   */
+  getExtension(format, extension) {
+    format = format.toLowerCase();
+    extension = extension.toLowerCase();
+    const key = `${format}|${extension}`;
+    return this.extensionCache_[key] || null;
+  }
+
+  checkEntityTransformed_(entity, transformed) {
+    const isEnabled = this.isEnabled_(entity, 'transformed');
+    const isDisabled = this.isDisabled_(entity, 'transformed');
+    if (transformed) {
+      return isEnabled !== false && isDisabled !== true;
+    }
+    return isEnabled !== true && isDisabled !== false;
+  }
+
+  checkEntityFormat_(entity, format) {
+    format = format.toLowerCase();
+    const isEnabled = this.isEnabled_(entity, format);
+    const isDisabled = this.isDisabled_(entity, format);
+    return isEnabled !== false && isDisabled !== true;
+  }
+
+  isEnabled_(entity, format) {
+    if (!entity.enabledBy) {
+      return null;
+    }
+    return entity.enabledBy.includes(format);
+  }
+
+  isDisabled_(entity, format) {
+    if (!entity.disabledBy) {
+      return null;
+    }
+    return entity.disabledBy.includes(format);
+  }
+
+  initRules_(rules) {
+    this.initErrors_(rules);
+    this.initAttrLists_(rules);
+    this.initTags_(rules);
+    this.initExtensions_(rules);
+  }
+
+  initErrors_(rules) {
+    this.errors = {};
+    for (const errorFormat of rules.errorFormats) {
+      const error = this.errors[errorFormat.code] || {};
+      error.format = errorFormat.format;
+      this.errors[errorFormat.code] = error;
+    }
+    for (const errorSpecificity of rules.errorSpecificity) {
+      const error = this.errors[errorSpecificity.code] || {};
+      error.specificity = errorSpecificity.specificity;
+      this.errors[errorSpecificity.code] = error;
+    }
+  }
+
+  initAttrLists_(rules) {
+    this.attrLists_ = {};
+    this.specialAttrLists_ = {};
+    for (const {name, attrs} of rules.attrLists) {
+      if (name.startsWith('$')) {
+        this.specialAttrLists_[name] = attrs;
+      } else {
+        this.attrLists_[name] = attrs;
+      }
+    }
+    this.specialAttrLists_.$AMP_LAYOUT_ATTRS.forEach(
+        (attr) => (attr.layout = true)
+    );
+    this.specialAttrLists_.$GLOBAL_ATTRS.forEach((attr) => (attr.global = true));
+  }
+
+  initTags_(rules) {
+    this.tags = rules.tags
+        .filter((tag) => !tag.extensionSpec)
+        .map((tag) => {
+          tag.attrs = tag.attrs || [];
+
+          // `attrLists` contains list IDs that are looked up from the global
+          // attribute lists and merged into `attrs`.
+          if (tag.attrLists) {
+            for (const attrList of tag.attrLists) {
+              tag.attrs.push(...this.attrLists_[attrList]);
+            }
+            delete tag.attrLists;
+          }
+
+          // $AMP_LAYOUT_ATTRS are present in all components with ampLayout
+          if (tag.ampLayout) {
+            tag.attrs.push(...this.specialAttrLists_.$AMP_LAYOUT_ATTRS);
+          }
+
+          // $GLOBAL_ATTRS are present in all components
+          tag.attrs.push(...this.specialAttrLists_.$GLOBAL_ATTRS);
+
+          return tag;
+        });
+  }
+
+  initExtensions_(rules) {
+    this.extensions = rules.tags
+        .filter((tag) => tag.extensionSpec)
+        .map((tag) =>
+          Object.assign({}, tag.extensionSpec, {htmlFormat: tag.htmlFormat})
+        );
+
+    for (const extension of this.extensions) {
+      const name = extension.name.toLowerCase();
+      for (let format of extension.htmlFormat) {
+        format = format.toLowerCase();
+        const key = `${format}|${name}`;
+        this.extensionCache_[key] = extension;
+      }
+    }
+  }
+}
+
+module.exports = AmpValidatorRules;
diff --git a/packages/validator-rules/lib/loadRules.js b/packages/validator-rules/lib/loadRules.js
new file mode 100644
index 000000000..40f3bf917
--- /dev/null
+++ b/packages/validator-rules/lib/loadRules.js
@@ -0,0 +1,34 @@
+/**
+ * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// This solution is temporary and will be replaced when
+// https://github.com/ampproject/amp-toolbox/issues/378 is resolved.
+
+const {oneBehindFetch} = require('@ampproject/toolbox-core');
+
+const VALIDATOR_RULES_URL = 'https://cdn.ampproject.org/v0/validator.json';
+
+async function loadRemote(url) {
+  const req = await oneBehindFetch(url);
+  return req.json();
+}
+
+async function loadRules({url}) {
+  url = url || VALIDATOR_RULES_URL;
+  return loadRemote(url);
+}
+
+module.exports = loadRules;
diff --git a/packages/validator-rules/package.json b/packages/validator-rules/package.json
new file mode 100644
index 000000000..5e9b756fe
--- /dev/null
+++ b/packages/validator-rules/package.json
@@ -0,0 +1,30 @@
+{
+  "name": "@ampproject/toolbox-validator-rules",
+  "version": "1.0.0-beta.3",
+  "description": "A library that helps query AMP Validator rules",
+  "main": "index.js",
+  "keywords": [
+    "amp"
+  ],
+  "scripts": {
+    "fetchRules": "curl https://cdn.ampproject.org/v0/validator.json --output validator.json"
+  },
+  "files": [
+    "index.js",
+    "lib"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/ampproject/amp-toolbox.git"
+  },
+  "author": "AMPHTML Team",
+  "license": "Apache-2.0",
+  "bugs": {
+    "url": "https://github.com/ampproject/amp-toolbox/issues"
+  },
+  "dependencies": {
+    "@ampproject/toolbox-core": "^1.0.0-beta.3"
+  },
+  "homepage": "https://github.com/ampproject/amp-toolbox/tree/master/packages/validator-rules",
+  "gitHead": "cb496f081c871a25dc73113ae1e1fccfb59b2732"
+}
diff --git a/packages/validator-rules/spec/AmpValidatorRulesSpec.js b/packages/validator-rules/spec/AmpValidatorRulesSpec.js
new file mode 100644
index 000000000..820b098e9
--- /dev/null
+++ b/packages/validator-rules/spec/AmpValidatorRulesSpec.js
@@ -0,0 +1,170 @@
+/**
+ * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+const AmpValidatorRules = require('../lib/AmpValidatorRules');
+
+describe('AmpValidatorRules', () => {
+  it('Loads errors', () => {
+    const rules = makeRules({
+      errorFormats: [
+        {
+          code: 'TEST',
+          format: '%s error',
+        },
+      ],
+      errorSpecificity: [
+        {
+          code: 'TEST',
+          specificity: 1,
+        },
+      ],
+    });
+    expect(rules.errors).toEqual({
+      TEST: {
+        format: '%s error',
+        specificity: 1,
+      },
+    });
+  });
+
+  it('Loads extensions', () => {
+    const rules = makeRules({
+      tags: [
+        {
+          extensionSpec: {
+            name: 'amp-some-component',
+            version: ['0.1', 'latest'],
+          },
+          htmlFormat: ['AMP'],
+          tagName: 'SCRIPT',
+        },
+      ],
+    });
+    expect(rules.tags).toEqual([]);
+    expect(rules.extensions).toEqual([
+      {
+        name: 'amp-some-component',
+        version: ['0.1', 'latest'],
+        htmlFormat: ['AMP'],
+      },
+    ]);
+    expect(rules.getExtension('AMP', 'amp-some-component')).toEqual({
+      name: 'amp-some-component',
+      version: ['0.1', 'latest'],
+      htmlFormat: ['AMP'],
+    });
+    expect(rules.getExtension('AMP4EMAIL', 'amp-some-component')).toEqual(null);
+  });
+
+  it('Loads tags', () => {
+    const rules = makeRules({
+      attrLists: [
+        {
+          name: '$GLOBAL_ATTRS',
+          attrs: [{name: 'global'}],
+        },
+        {
+          name: '$AMP_LAYOUT_ATTRS',
+          attrs: [{name: 'layoutattr'}],
+        },
+        {
+          name: 'some-list',
+          attrs: [{name: 'test'}],
+        },
+      ],
+      tags: [
+        {
+          htmlFormat: ['AMP', 'AMP4EMAIL'],
+          attrs: [{name: 'align'}],
+          attrLists: ['some-list'],
+          tagName: 'DIV',
+        },
+        {
+          htmlFormat: ['AMP', 'AMP4EMAIL'],
+          attrs: [{name: 'align'}],
+          disabledBy: ['transformed'],
+          ampLayout: {
+            supportedLayouts: ['FIXED', 'FIXED_HEIGHT'],
+          },
+          tagName: 'AMP-IMG',
+        },
+      ],
+    });
+
+    const tags = [
+      {
+        htmlFormat: ['AMP', 'AMP4EMAIL'],
+        attrs: [
+          {name: 'align'},
+          {name: 'test'},
+          {
+            name: 'global',
+            global: true,
+          },
+        ],
+        tagName: 'DIV',
+      },
+      {
+        htmlFormat: ['AMP', 'AMP4EMAIL'],
+        attrs: [
+          {name: 'align'},
+          {
+            name: 'layoutattr',
+            layout: true,
+          },
+          {
+            name: 'global',
+            global: true,
+          },
+        ],
+        disabledBy: ['transformed'],
+        ampLayout: {
+          supportedLayouts: ['FIXED', 'FIXED_HEIGHT'],
+        },
+        tagName: 'AMP-IMG',
+      },
+    ];
+    expect(rules.tags).toEqual(tags);
+    expect(rules.extensions).toEqual([]);
+    expect(rules.getTagsForFormat('AMP4EMAIL')).toEqual(tags);
+    expect(rules.getTagsForFormat('AMP', true)).toEqual([tags[0]]);
+  });
+});
+
+function makeRules(rules) {
+  return new AmpValidatorRules(
+      Object.assign(
+          {
+            errorFormats: [],
+            errorSpecificity: [],
+            attrLists: [
+              {
+                name: '$AMP_LAYOUT_ATTRS',
+                attrs: [],
+              },
+              {
+                name: '$GLOBAL_ATTRS',
+                attrs: [],
+              },
+            ],
+            tags: [],
+          },
+          rules
+      )
+  );
+}