diff --git a/qutebrowser/browser/greasemonkey.py b/qutebrowser/browser/greasemonkey.py
index 7ef1f0ff2..07fa8d581 100644
--- a/qutebrowser/browser/greasemonkey.py
+++ b/qutebrowser/browser/greasemonkey.py
@@ -31,7 +31,8 @@ import attr
 from PyQt5.QtCore import pyqtSignal, QObject, QUrl
 
 from qutebrowser.utils import (log, standarddir, jinja, objreg, utils,
-                               javascript, urlmatch, version, usertypes)
+                               javascript, urlmatch, version, usertypes,
+                               qtutils)
 from qutebrowser.api import cmdutils
 from qutebrowser.browser import downloads
 from qutebrowser.misc import objects
@@ -116,6 +117,40 @@ class GreasemonkeyScript:
             script.includes = ['*']
         return script
 
+    def force_document_end(self):
+        """Check whether to force @run-at document-end.
+
+        This needs to be done on QtWebEngine with Qt 5.12 for known-broken
+        scripts.
+
+        On Qt 5.12, accessing the DOM isn't possible with "@run-at
+        document-start". It was documented to be impossible before, but seems
+        to work fine.
+
+        However, some scripts do DOM access with "@run-at document-start". Fix
+        those by forcing them to use document-end instead.
+        """
+        if objects.backend != usertypes.Backend.QtWebEngine:
+            return False
+        elif not qtutils.version_check('5.12', compiled=False):
+            return False
+
+        broken_scripts = [
+            ('http://userstyles.org', None),
+            ('https://github.com/ParticleCore', 'Iridium'),
+        ]
+        return any(self._matches_id(namespace=namespace, name=name)
+                   for namespace, name in broken_scripts)
+
+    def _matches_id(self, *, namespace, name):
+        """Check if this script matches the given namespace/name.
+
+        Both namespace and name can be None in order to match any script.
+        """
+        matches_namespace = namespace is None or self.namespace == namespace
+        matches_name = name is None or self.name == name
+        return matches_namespace and matches_name
+
     def code(self):
         """Return the processed JavaScript code of this script.
 
diff --git a/qutebrowser/browser/webengine/webenginetab.py b/qutebrowser/browser/webengine/webenginetab.py
index 88c283126..41081abb8 100644
--- a/qutebrowser/browser/webengine/webenginetab.py
+++ b/qutebrowser/browser/webengine/webenginetab.py
@@ -1038,9 +1038,15 @@ class _WebEngineScripts(QObject):
             new_script.setSourceCode(script.code())
             new_script.setName("GM-{}".format(script.name))
             new_script.setRunsOnSubFrames(script.runs_on_sub_frames)
+
             # Override the @run-at value parsed by QWebEngineScript if desired.
             if injection_point:
                 new_script.setInjectionPoint(injection_point)
+            elif script.force_document_end():
+                log.greasemonkey.debug("Forcing @run-at document-end for {}"
+                                       .format(script.name))
+                new_script.setInjectionPoint(QWebEngineScript.DocumentReady)
+
             log.greasemonkey.debug('adding script: {}'
                                    .format(new_script.name()))
             page_scripts.insert(new_script)
diff --git a/tests/unit/browser/webengine/test_webenginetab.py b/tests/unit/browser/webengine/test_webenginetab.py
index 251abb84a..79ac8d37a 100644
--- a/tests/unit/browser/webengine/test_webenginetab.py
+++ b/tests/unit/browser/webengine/test_webenginetab.py
@@ -25,8 +25,10 @@ import pytest
 QtWebEngineWidgets = pytest.importorskip("PyQt5.QtWebEngineWidgets")
 QWebEnginePage = QtWebEngineWidgets.QWebEnginePage
 QWebEngineScriptCollection = QtWebEngineWidgets.QWebEngineScriptCollection
+QWebEngineScript = QtWebEngineWidgets.QWebEngineScript
 
 from qutebrowser.browser import greasemonkey
+from qutebrowser.utils import usertypes
 
 pytestmark = pytest.mark.usefixtures('greasemonkey_manager')
 
@@ -91,3 +93,26 @@ class TestWebengineScripts:
 
         collection = webengine_scripts._widget.page().scripts()
         assert collection.toList()[-1].worldId() == worldid
+
+    def test_greasemonkey_force_document_end(self, monkeypatch,
+                                             webengine_scripts):
+        """Make sure document-end is forced when needed."""
+        monkeypatch.setattr(greasemonkey.objects, 'backend',
+                            usertypes.Backend.QtWebEngine)
+        monkeypatch.setattr(greasemonkey.qtutils, 'version_check',
+                            lambda version, exact=False, compiled=True:
+                            True)
+
+        scripts = [
+            greasemonkey.GreasemonkeyScript([
+                ('name', 'Iridium'),
+                ('namespace', 'https://github.com/ParticleCore'),
+                ('run-at', 'document-start'),
+            ], None)
+        ]
+
+        webengine_scripts._inject_greasemonkey_scripts(scripts)
+
+        collection = webengine_scripts._widget.page().scripts()
+        script = collection.toList()[-1]
+        assert script.injectionPoint() == QWebEngineScript.DocumentReady
diff --git a/tests/unit/javascript/test_greasemonkey.py b/tests/unit/javascript/test_greasemonkey.py
index 6c1c35e06..48d2e075c 100644
--- a/tests/unit/javascript/test_greasemonkey.py
+++ b/tests/unit/javascript/test_greasemonkey.py
@@ -25,6 +25,7 @@ import pytest
 import py.path  # pylint: disable=no-name-in-module
 from PyQt5.QtCore import QUrl
 
+from qutebrowser.utils import usertypes
 from qutebrowser.browser import greasemonkey
 
 test_gm_script = r"""
@@ -165,6 +166,56 @@ def test_utf8_bom():
     assert '// ==UserScript==' in script.code().splitlines()
 
 
+class TestForceDocumentEnd:
+
+    @pytest.fixture
+    def patch(self, monkeypatch):
+        def _patch(*, backend, qt_512):
+            monkeypatch.setattr(greasemonkey.objects, 'backend', backend)
+            monkeypatch.setattr(greasemonkey.qtutils, 'version_check',
+                                lambda version, exact=False, compiled=True:
+                                qt_512)
+        return _patch
+
+    def _get_script(self, *, namespace, name):
+        source = textwrap.dedent("""
+            // ==UserScript==
+            // @namespace {}
+            // @name {}
+            // ==/UserScript==
+        """.format(namespace, name))
+        _save_script(source, 'force.user.js')
+
+        gm_manager = greasemonkey.GreasemonkeyManager()
+
+        scripts = gm_manager.all_scripts()
+        assert len(scripts) == 1
+        return scripts[0]
+
+    @pytest.mark.parametrize('backend, qt_512', [
+        (usertypes.Backend.QtWebKit, True),
+        (usertypes.Backend.QtWebEngine, False),
+    ])
+    def test_not_applicable(self, patch, backend, qt_512):
+        """Test backend/Qt version combinations which don't need a fix."""
+        patch(backend=backend, qt_512=qt_512)
+        script = self._get_script(namespace='https://github.com/ParticleCore',
+                                  name='Iridium')
+        assert not script.force_document_end()
+
+    @pytest.mark.parametrize('namespace, name, force', [
+        ('http://userstyles.org', 'foobar', True),
+        ('https://github.com/ParticleCore', 'Iridium', True),
+        ('https://github.com/ParticleCore', 'Foo', False),
+        ('https://example.org', 'Iridium', False),
+    ])
+    def test_matching(self, patch, namespace, name, force):
+        """Test matching based on namespace/name."""
+        patch(backend=usertypes.Backend.QtWebEngine, qt_512=True)
+        script = self._get_script(namespace=namespace, name=name)
+        assert script.force_document_end() == force
+
+
 def test_required_scripts_are_included(download_stub, tmpdir):
     test_require_script = textwrap.dedent("""
         // ==UserScript==