--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -286,16 +286,25 @@ class WPTManifestUpdater(MozbuildObject)
     def run_update(self, check_clean=False, rebuild=False, **kwargs):
         import manifestupdate
         from wptrunner import wptlogging
         logger = wptlogging.setup(kwargs, {"mach": sys.stdout})
         wpt_dir = os.path.abspath(os.path.join(self.topsrcdir, 'testing', 'web-platform'))
         manifestupdate.update(logger, wpt_dir, check_clean, rebuild)
 
 
+class WPTManifestDownloader(MozbuildObject):
+    def run_download(self, path=None, tests_root=None, force=False, **kwargs):
+        import manifestdownload
+        from wptrunner import wptlogging
+        logger = wptlogging.setup(kwargs, {"mach": sys.stdout})
+        wpt_dir = os.path.abspath(os.path.join(self.topsrcdir, 'testing', 'web-platform'))
+        manifestdownload.run(logger, wpt_dir, self.topsrcdir, force)
+
+
 def create_parser_update():
     from update import updatecommandline
     return updatecommandline.create_parser()
 
 def create_parser_reduce():
     from wptrunner import wptcommandline
     return wptcommandline.create_parser_reduce()
 
@@ -319,16 +328,20 @@ def create_parser_create():
     p.add_argument("path", action="store", help="Path to the test file")
     return p
 
 
 def create_parser_manifest_update():
     import manifestupdate
     return manifestupdate.create_parser()
 
+def create_parser_manifest_download():
+    import manifestdownload
+    return manifestdownload.create_parser()
+
 
 @CommandProvider
 class MachCommands(MachCommandBase):
     def setup(self):
         self._activate_virtualenv()
 
     @Command("web-platform-tests",
              category="testing",
@@ -402,8 +415,17 @@ class MachCommands(MachCommandBase):
 
     @Command("wpt-manifest-update",
              category="testing",
              parser=create_parser_manifest_update)
     def wpt_manifest_update(self, **params):
         self.setup()
         wpt_manifest_updater = self._spawn(WPTManifestUpdater)
         return wpt_manifest_updater.run_update(**params)
+
+
+    @Command("wpt-manifest-download",
+             category="testing",
+             parser=create_parser_manifest_download)
+    def wpt_manifest_download(self, **params):
+        self.setup()
+        wpt_manifest_downloader = self._spawn(WPTManifestDownloader)
+        return wpt_manifest_downloader.run_download(**params)

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/manifestdownload.py
@@ -0,0 +1,130 @@
+from __future__ import absolute_import
+
+import argparse
+import json
+import io
+import os
+from datetime import datetime, timedelta
+import gzip
+from vcs import Mercurial
+import requests
+
+from six.moves.urllib.request import urlopen
+
+def abs_path(path):
+    return os.path.abspath(os.path.expanduser(path))
+
+
+def hg_commits(repo_root):
+    hg = Mercurial.get_func(repo_root)
+    return [item for item in hg("log", "-fl50", "--template={node}\n",
+            "testing/web-platform/tests/", "testing/web-platform/mozilla/tests").split("\n")
+            if item]
+
+
+def should_download(logger, manifest_path, rebuild_time=timedelta(days=5)):
+    # TODO: Improve logic for when to download. Maybe if x revisions behind?
+    if not os.path.exists(manifest_path):
+        return True
+    mtime = datetime.fromtimestamp(os.path.getmtime(manifest_path))
+    if mtime < datetime.now() - rebuild_time:
+        return True
+    logger.info("Skipping manifest download because existing file is recent")
+    return False
+
+
+def taskcluster_url(logger, commits):
+    cset_url = ('https://hg.mozilla.org/mozilla-central/json-pushes?'
+                'changeset={changeset}&version=2&tipsonly=1')
+
+    tc_url = ('https://index.taskcluster.net/v1/task/gecko.v2.mozilla-central.'
+              'revision.{changeset}.source.manifest-upload')
+
+    for revision in commits:
+        req = requests.get(cset_url.format(changeset=revision),
+                           headers={'Accept': 'application/json'})
+
+        req.raise_for_status()
+
+        result = req.json()
+        [cset] = result['pushes'].values()[0]['changesets']
+        resp = requests.get(tc_url.format(changeset=cset))
+
+        if req.status_code == 200:
+            return tc_url.format(changeset=cset)
+
+    logger.info("Can't find a commit-specific manifest so just using the most"
+                "recent one")
+
+    return ("https://index.taskcluster.net/v1/task/gecko.v2.mozilla-central."
+            "latest.source.manifest-upload")
+
+
+def download_manifest(logger, wpt_dir, commits_func, url_func, force=False):
+    if not force and not should_download(logger, wpt_dir):
+        return False
+
+    commits = commits_func()
+    url = url_func(logger, commits) + "/artifacts/public/"
+
+    man_url= url + "manifest.json.gz"
+    moz_man_url= url + "moz_manifest.json.gz"
+
+    return ( _download(logger, os.path.join(wpt_dir, "meta", "MANIFEST.json"), man_url) and
+             _download(logger, os.path.join(wpt_dir,"mozilla", "meta", "MANIFEST.json"), moz_man_url))
+
+
+def _download(logger, manifest_path, url):
+    if not url:
+        logger.warning("No generated manifest found")
+        return False
+
+    logger.info("Downloading manifest from %s" % url)
+    try:
+        resp = urlopen(url)
+    except Exception:
+        logger.warning("Downloading pregenerated manifest failed")
+        return False
+
+    if resp.code != 200:
+        logger.warning("Downloading pregenerated manifest failed; got"
+                        "HTTP status %d" % resp.code)
+        return False
+
+    gzf = gzip.GzipFile(fileobj=io.BytesIO(resp.read()))
+
+    try:
+        decompressed = gzf.read()
+    except IOError:
+        logger.warning("Failed to decompress downloaded file")
+        return False
+
+    try:
+        with open(manifest_path, 'wb') as f:
+            f.write(decompressed)
+    except Exception as e:
+        logger.warning("Failed to write manifest at %s" % manifest_path)
+        return False
+
+    logger.info("Manifest at %s downloaded" % manifest_path)
+    return True
+
+
+def create_parser():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "-p", "--path", type=abs_path, help="Path to manifest file.")
+    parser.add_argument(
+        "--force", action="store_true",
+        help="Always download, even if the existing manifest is recent")
+    return parser
+
+
+def download_from_taskcluster(logger, wpt_dir, repo_root, force=False):
+    return download_manifest(logger, wpt_dir, lambda: hg_commits(repo_root),
+                             taskcluster_url, force)
+
+
+def run(logger, wpt_dir, repo_root, force=False):
+    success = download_from_taskcluster(logger, wpt_dir, repo_root, force)
+    return 0 if success else 1

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/vcs.py
@@ -0,0 +1,20 @@
+import os
+import subprocess
+
+class Mercurial(object):
+
+    def __init__(self, repo_root):
+        self.root = os.path.abspath(repo_root)
+        self.hg = Mercurial.get_func(repo_root)
+
+
+    @staticmethod
+    def get_func(repo_path):
+        def hg(cmd, *args):
+            full_cmd = ["hg", cmd] + list(args)
+            try:
+                return subprocess.check_output(full_cmd, cwd=repo_path, stderr=subprocess.STDOUT)
+            except Exception as e:
+                raise(e)
+            # TODO: Test on Windows.
+        return hg