diff --git a/.github/workflows/tests-windows.yml b/.github/workflows/tests-windows.yml
index 30fda33e8..ab7385118 100644
--- a/.github/workflows/tests-windows.yml
+++ b/.github/workflows/tests-windows.yml
@@ -17,10 +17,15 @@ jobs:
         - python-version: 3.8
           env:
             TOXENV: py
-        # https://twistedmatrix.com/trac/ticket/9990
-        #- python-version: 3.9
-          #env:
-            #TOXENV: py
+        - python-version: 3.9
+          env:
+            TOXENV: py
+        - python-version: "3.10"
+          env:
+            TOXENV: py
+        - python-version: "3.10"
+          env:
+            TOXENV: asyncio
 
     steps:
     - uses: actions/checkout@v2
diff --git a/docs/conftest.py b/docs/conftest.py
index 8c735e838..a0636f8ac 100644
--- a/docs/conftest.py
+++ b/docs/conftest.py
@@ -3,7 +3,11 @@ from doctest import ELLIPSIS, NORMALIZE_WHITESPACE
 
 from scrapy.http.response.html import HtmlResponse
 from sybil import Sybil
-from sybil.parsers.codeblock import CodeBlockParser
+try:
+    # >2.0.1
+    from sybil.parsers.codeblock import PythonCodeBlockParser
+except ImportError:
+    from sybil.parsers.codeblock import CodeBlockParser as PythonCodeBlockParser
 from sybil.parsers.doctest import DocTestParser
 from sybil.parsers.skip import skip
 
@@ -21,7 +25,7 @@ def setup(namespace):
 pytest_collect_file = Sybil(
     parsers=[
         DocTestParser(optionflags=ELLIPSIS | NORMALIZE_WHITESPACE),
-        CodeBlockParser(future_imports=['print_function']),
+        PythonCodeBlockParser(future_imports=['print_function']),
         skip,
     ],
     pattern='*.rst',
diff --git a/docs/news.rst b/docs/news.rst
index 509366c17..2afe318f6 100644
--- a/docs/news.rst
+++ b/docs/news.rst
@@ -1,3 +1,25 @@
+.. note::
+    .. versionchanged:: VERSION
+
+    The Twisted reactor is now installed when
+    :meth:`~scrapy.crawler.CrawlerProcess.crawl` is first called, not when a
+    :class:`scrapy.crawler.CrawlerProcess` object is created. Because of this,
+    :setting:`TWISTED_REACTOR` and :setting:`ASYNCIO_EVENT_LOOP` are now
+    honored in :attr:`~scrapy.Spider.custom_settings`. In older Scrapy versions
+    they are silently ignored when set there and you need to set these settings
+    in some other way.
+
+
+.. note::
+    .. versionchanged:: VERSION
+
+    Previously this setting had no effect in a spider
+    :attr:`~scrapy.Spider.custom_settings` attribute. Now it will be used, but
+    if you :ref:`run several spiders in one process <run-multiple-spiders>`,
+    they must not have different values for this setting, because they will use
+    a single reactor instance.
+
+
 .. _news:
 
 Release notes
diff --git a/docs/topics/architecture.rst b/docs/topics/architecture.rst
index 71d027c86..0c3a7ed88 100644
--- a/docs/topics/architecture.rst
+++ b/docs/topics/architecture.rst
@@ -67,7 +67,7 @@ this:
    the :ref:`Scheduler <component-scheduler>` and asks for possible next Requests
    to crawl.
 
-9. The process repeats (from step 1) until there are no more requests from the
+9. The process repeats (from step 3) until there are no more requests from the
    :ref:`Scheduler <component-scheduler>`.
 
 Components
diff --git a/docs/topics/asyncio.rst b/docs/topics/asyncio.rst
index 28241ae24..8712d4268 100644
--- a/docs/topics/asyncio.rst
+++ b/docs/topics/asyncio.rst
@@ -10,11 +10,6 @@ Scrapy has partial support for :mod:`asyncio`. After you :ref:`install the
 asyncio reactor <install-asyncio>`, you may use :mod:`asyncio` and
 :mod:`asyncio`-powered libraries in any :doc:`coroutine <coroutines>`.
 
-.. warning:: :mod:`asyncio` support in Scrapy is experimental, and not yet
-             recommended for production environments. Future Scrapy versions
-             may introduce related changes without a deprecation period or
-             warning.
-
 .. _install-asyncio:
 
 Installing the asyncio reactor
@@ -41,6 +36,34 @@ use it instead of the default asyncio event loop.
 
 .. _asyncio-await-dfd:
 
+Windows-specific notes
+======================
+
+The Windows implementation of :mod:`asyncio` can use two event loop
+implementations: :class:`~asyncio.SelectorEventLoop` (default before Python
+3.8, required when using Twisted) and :class:`~asyncio.ProactorEventLoop`
+(default since Python 3.8, cannot work with Twisted). So on Python 3.8+ the
+event loop class needs to be changed. Scrapy since VERSION does this
+automatically when you change the :setting:`TWISTED_REACTOR` setting or call
+:func:`~scrapy.utils.reactor.install_reactor`, but if you install the reactor
+by other means or use an older Scrapy version you need to call the following
+code before installing the reactor::
+
+    import asyncio
+    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
+
+You can put this in the same function that installs the reactor, if you do that
+yourself, or in some code that runs before the reactor is installed, e.g.
+``settings.py``.
+
+.. note:: Other libraries you use may require
+          :class:`~asyncio.ProactorEventLoop`, e.g. because it supports
+          subprocesses (this is the case with `playwright`_), so you cannot use
+          them together with Scrapy on Windows (but you should be able to use
+          them on WSL or native Linux).
+
+.. _playwright: https://github.com/microsoft/playwright-python
+
 Awaiting on Deferreds
 =====================
 
diff --git a/docs/topics/practices.rst b/docs/topics/practices.rst
index 732eba587..1a9d56143 100644
--- a/docs/topics/practices.rst
+++ b/docs/topics/practices.rst
@@ -193,6 +193,25 @@ Same example but running the spiders sequentially by chaining the deferreds:
     crawl()
     reactor.run() # the script will block here until the last crawl call is finished
 
+Different spiders can set different values for the same setting, but when they
+run in the same process it may be impossible, by design or because of some
+limitations, to use these different values. What happens in practice is
+different for different settings:
+
+* :setting:`SPIDER_LOADER_CLASS` and the ones used by its value
+  (:setting:`SPIDER_MODULES`, :setting:`SPIDER_LOADER_WARN_ONLY` for the
+  default one) cannot be read from the per-spider settings. These are applied
+  when the :class:`~scrapy.crawler.CrawlerRunner` or
+  :class:`~scrapy.crawler.CrawlerProcess` object is created.
+* For :setting:`TWISTED_REACTOR` and :setting:`ASYNCIO_EVENT_LOOP` the first
+  available value is used, and if a spider requests a different reactor an
+  exception will be raised. These are applied when the reactor is installed.
+* For :setting:`REACTOR_THREADPOOL_MAXSIZE`, :setting:`DNS_RESOLVER` and the
+  ones used by the resolver (:setting:`DNSCACHE_ENABLED`,
+  :setting:`DNSCACHE_SIZE`, :setting:`DNS_TIMEOUT` for ones included in Scrapy)
+  the first available value is used. These are applied when the reactor is
+  started.
+
 .. seealso:: :ref:`run-from-script`.
 
 .. _distributed-crawls:
diff --git a/docs/topics/settings.rst b/docs/topics/settings.rst
index 210c1def7..f6c95c502 100644
--- a/docs/topics/settings.rst
+++ b/docs/topics/settings.rst
@@ -1638,10 +1638,9 @@ which raises :exc:`Exception`, becomes::
 
 
 The default value of the :setting:`TWISTED_REACTOR` setting is ``None``, which
-means that Scrapy will not attempt to install any specific reactor, and the
-default reactor defined by Twisted for the current platform will be used. This
-is to maintain backward compatibility and avoid possible problems caused by
-using a non-default reactor.
+means that Scrapy will install the default reactor defined by Twisted for the
+current platform. This is to maintain backward compatibility and avoid possible
+problems caused by using a non-default reactor.
 
 For additional information, see :doc:`core/howto/choosing-reactor`.
 
diff --git a/pylintrc b/pylintrc
index 699686e16..2cdd6321e 100644
--- a/pylintrc
+++ b/pylintrc
@@ -112,6 +112,7 @@ disable=abstract-method,
         unused-private-member,
         unused-variable,
         unused-wildcard-import,
+        use-implicit-booleaness-not-comparison,
         used-before-assignment,
         useless-object-inheritance,  # Required for Python 2 support
         useless-return,
diff --git a/scrapy/commands/__init__.py b/scrapy/commands/__init__.py
index 6e77551c6..5f1dabd33 100644
--- a/scrapy/commands/__init__.py
+++ b/scrapy/commands/__init__.py
@@ -43,14 +43,14 @@ class ScrapyCommand:
 
     def long_desc(self):
         """A long description of the command. Return short description when not
-        available. It cannot contain newlines, since contents will be formatted
+        available. It cannot contain newlines since contents will be formatted
         by optparser which removes newlines and wraps text.
         """
         return self.short_desc()
 
     def help(self):
         """An extensive help for the command. It will be shown when using the
-        "help" command. It can contain newlines, since no post-formatting will
+        "help" command. It can contain newlines since no post-formatting will
         be applied to its contents.
         """
         return self.long_desc()
diff --git a/scrapy/commands/shell.py b/scrapy/commands/shell.py
index d1944df3d..de81986d8 100644
--- a/scrapy/commands/shell.py
+++ b/scrapy/commands/shell.py
@@ -75,6 +75,6 @@ class Command(ScrapyCommand):
 
     def _start_crawler_thread(self):
         t = Thread(target=self.crawler_process.start,
-                   kwargs={'stop_after_crawl': False})
+                   kwargs={'stop_after_crawl': False, 'install_signal_handlers': False})
         t.daemon = True
         t.start()
diff --git a/scrapy/core/downloader/handlers/http11.py b/scrapy/core/downloader/handlers/http11.py
index 8a91d4c5e..38935667d 100644
--- a/scrapy/core/downloader/handlers/http11.py
+++ b/scrapy/core/downloader/handlers/http11.py
@@ -213,7 +213,7 @@ class TunnelingAgent(Agent):
         # proxy host and port are required for HTTP pool `key`
         # otherwise, same remote host connection request could reuse
         # a cached tunneled connection to a different proxy
-        key = key + self._proxyConf
+        key += self._proxyConf
         return super()._requestWithEndpoint(
             key=key,
             endpoint=endpoint,
diff --git a/scrapy/core/downloader/middleware.py b/scrapy/core/downloader/middleware.py
index a5619d8a4..289147466 100644
--- a/scrapy/core/downloader/middleware.py
+++ b/scrapy/core/downloader/middleware.py
@@ -3,7 +3,7 @@ Downloader Middleware manager
 
 See documentation in docs/topics/downloader-middleware.rst
 """
-from typing import Callable, Union
+from typing import Callable, Union, cast
 
 from twisted.internet import defer
 from twisted.python.failure import Failure
@@ -37,6 +37,7 @@ class DownloaderMiddlewareManager(MiddlewareManager):
         @defer.inlineCallbacks
         def process_request(request: Request):
             for method in self.methods['process_request']:
+                method = cast(Callable, method)
                 response = yield deferred_from_coro(method(request=request, spider=spider))
                 if response is not None and not isinstance(response, (Response, Request)):
                     raise _InvalidOutput(
@@ -55,6 +56,7 @@ class DownloaderMiddlewareManager(MiddlewareManager):
                 return response
 
             for method in self.methods['process_response']:
+                method = cast(Callable, method)
                 response = yield deferred_from_coro(method(request=request, response=response, spider=spider))
                 if not isinstance(response, (Response, Request)):
                     raise _InvalidOutput(
@@ -69,6 +71,7 @@ class DownloaderMiddlewareManager(MiddlewareManager):
         def process_exception(failure: Failure):
             exception = failure.value
             for method in self.methods['process_exception']:
+                method = cast(Callable, method)
                 response = yield deferred_from_coro(method(request=request, exception=exception, spider=spider))
                 if response is not None and not isinstance(response, (Response, Request)):
                     raise _InvalidOutput(
diff --git a/scrapy/core/downloader/webclient.py b/scrapy/core/downloader/webclient.py
index 915cb5fe3..06cb96489 100644
--- a/scrapy/core/downloader/webclient.py
+++ b/scrapy/core/downloader/webclient.py
@@ -3,7 +3,7 @@ from time import time
 from urllib.parse import urlparse, urlunparse, urldefrag
 
 from twisted.web.http import HTTPClient
-from twisted.internet import defer, reactor
+from twisted.internet import defer
 from twisted.internet.protocol import ClientFactory
 
 from scrapy.http import Headers
@@ -170,6 +170,7 @@ class ScrapyHTTPClientFactory(ClientFactory):
         p.followRedirect = self.followRedirect
         p.afterFoundGet = self.afterFoundGet
         if self.timeout:
+            from twisted.internet import reactor
             timeoutCall = reactor.callLater(self.timeout, p.timeout)
             self.deferred.addBoth(self._cancelTimeout, timeoutCall)
         return p
diff --git a/scrapy/core/http2/stream.py b/scrapy/core/http2/stream.py
index c2a4b702f..5c393c027 100644
--- a/scrapy/core/http2/stream.py
+++ b/scrapy/core/http2/stream.py
@@ -285,8 +285,8 @@ class Stream:
 
             self._protocol.conn.send_data(self.stream_id, data_chunk, end_stream=False)
 
-            bytes_to_send_size = bytes_to_send_size - chunk_size
-            self.metadata['remaining_content_length'] = self.metadata['remaining_content_length'] - chunk_size
+            bytes_to_send_size -= chunk_size
+            self.metadata['remaining_content_length'] -= chunk_size
 
         self.metadata['remaining_content_length'] = max(0, self.metadata['remaining_content_length'])
 
diff --git a/scrapy/core/spidermw.py b/scrapy/core/spidermw.py
index d1fedae07..009ece06d 100644
--- a/scrapy/core/spidermw.py
+++ b/scrapy/core/spidermw.py
@@ -4,7 +4,7 @@ Spider Middleware manager
 See documentation in docs/topics/spider-middleware.rst
 """
 from itertools import islice
-from typing import Any, AsyncGenerator, AsyncIterable, Callable, Generator, Iterable, Union
+from typing import Any, AsyncGenerator, AsyncIterable, Callable, Generator, Iterable, Union, cast
 
 from twisted.internet.defer import Deferred
 from twisted.python.failure import Failure
@@ -48,6 +48,7 @@ class SpiderMiddlewareManager(MiddlewareManager):
     def _process_spider_input(self, scrape_func: ScrapeFunc, response: Response, request: Request,
                               spider: Spider) -> Any:
         for method in self.methods['process_spider_input']:
+            method = cast(Callable, method)
             try:
                 result = method(response=response, spider=spider)
                 if result is not None:
diff --git a/scrapy/crawler.py b/scrapy/crawler.py
index 578016536..a638254f1 100644
--- a/scrapy/crawler.py
+++ b/scrapy/crawler.py
@@ -25,6 +25,7 @@ from scrapy.utils.log import (
     configure_logging,
     get_scrapy_root_handler,
     install_scrapy_root_handler,
+    log_reactor_info,
     log_scrapy_info,
     LogCounterHandler,
 )
@@ -38,7 +39,7 @@ logger = logging.getLogger(__name__)
 
 class Crawler:
 
-    def __init__(self, spidercls, settings=None):
+    def __init__(self, spidercls, settings=None, init_reactor: bool = False):
         if isinstance(spidercls, Spider):
             raise ValueError('The spidercls argument must be a class, not an object')
 
@@ -69,6 +70,20 @@ class Crawler:
 
         lf_cls = load_object(self.settings['LOG_FORMATTER'])
         self.logformatter = lf_cls.from_crawler(self)
+
+        reactor_class = self.settings.get("TWISTED_REACTOR")
+        if init_reactor:
+            # this needs to be done after the spider settings are merged,
+            # but before something imports twisted.internet.reactor
+            if reactor_class:
+                install_reactor(reactor_class, self.settings["ASYNCIO_EVENT_LOOP"])
+            else:
+                from twisted.internet import default
+                default.install()
+            log_reactor_info()
+        if reactor_class:
+            verify_installed_reactor(reactor_class)
+
         self.extensions = ExtensionManager.from_crawler(self)
 
         self.settings.freeze()
@@ -153,7 +168,6 @@ class CrawlerRunner:
         self._crawlers = set()
         self._active = set()
         self.bootstrap_failed = False
-        self._handle_twisted_reactor()
 
     @property
     def spiders(self):
@@ -247,10 +261,6 @@ class CrawlerRunner:
         while self._active:
             yield defer.DeferredList(self._active)
 
-    def _handle_twisted_reactor(self):
-        if self.settings.get("TWISTED_REACTOR"):
-            verify_installed_reactor(self.settings["TWISTED_REACTOR"])
-
 
 class CrawlerProcess(CrawlerRunner):
     """
@@ -278,7 +288,6 @@ class CrawlerProcess(CrawlerRunner):
 
     def __init__(self, settings=None, install_root_handler=True):
         super().__init__(settings)
-        install_shutdown_handlers(self._signal_shutdown)
         configure_logging(self.settings, install_root_handler)
         log_scrapy_info(self.settings)
 
@@ -298,7 +307,12 @@ class CrawlerProcess(CrawlerRunner):
                     {'signame': signame})
         reactor.callFromThread(self._stop_reactor)
 
-    def start(self, stop_after_crawl=True):
+    def _create_crawler(self, spidercls):
+        if isinstance(spidercls, str):
+            spidercls = self.spider_loader.load(spidercls)
+        return Crawler(spidercls, self.settings, init_reactor=True)
+
+    def start(self, stop_after_crawl=True, install_signal_handlers=True):
         """
         This method starts a :mod:`~twisted.internet.reactor`, adjusts its pool
         size to :setting:`REACTOR_THREADPOOL_MAXSIZE`, and installs a DNS cache
@@ -309,6 +323,9 @@ class CrawlerProcess(CrawlerRunner):
 
         :param bool stop_after_crawl: stop or not the reactor when all
             crawlers have finished
+
+        :param bool install_signal_handlers: whether to install the shutdown
+            handlers (default: True)
         """
         from twisted.internet import reactor
         if stop_after_crawl:
@@ -318,6 +335,8 @@ class CrawlerProcess(CrawlerRunner):
                 return
             d.addBoth(self._stop_reactor)
 
+        if install_signal_handlers:
+            install_shutdown_handlers(self._signal_shutdown)
         resolver_class = load_object(self.settings["DNS_RESOLVER"])
         resolver = create_instance(resolver_class, self.settings, self, reactor=reactor)
         resolver.install_on_reactor()
@@ -337,8 +356,3 @@ class CrawlerProcess(CrawlerRunner):
             reactor.stop()
         except RuntimeError:  # raised if already stopped or in shutdown stage
             pass
-
-    def _handle_twisted_reactor(self):
-        if self.settings.get("TWISTED_REACTOR"):
-            install_reactor(self.settings["TWISTED_REACTOR"], self.settings["ASYNCIO_EVENT_LOOP"])
-        super()._handle_twisted_reactor()
diff --git a/scrapy/middleware.py b/scrapy/middleware.py
index bbec38086..2eb1d8609 100644
--- a/scrapy/middleware.py
+++ b/scrapy/middleware.py
@@ -1,7 +1,7 @@
 import logging
 import pprint
 from collections import defaultdict, deque
-from typing import Callable, Deque, Dict
+from typing import Callable, Deque, Dict, Optional, cast, Iterable
 
 from twisted.internet.defer import Deferred
 
@@ -9,7 +9,7 @@ from scrapy import Spider
 from scrapy.exceptions import NotConfigured
 from scrapy.settings import Settings
 from scrapy.utils.misc import create_instance, load_object
-from scrapy.utils.defer import process_parallel, process_chain, process_chain_both
+from scrapy.utils.defer import process_parallel, process_chain
 
 logger = logging.getLogger(__name__)
 
@@ -21,7 +21,8 @@ class MiddlewareManager:
 
     def __init__(self, *middlewares):
         self.middlewares = middlewares
-        self.methods: Dict[str, Deque[Callable]] = defaultdict(deque)
+        # Optional because process_spider_output and process_spider_exception can be None
+        self.methods: Dict[str, Deque[Optional[Callable]]] = defaultdict(deque)
         for mw in middlewares:
             self._add_middleware(mw)
 
@@ -64,14 +65,12 @@ class MiddlewareManager:
             self.methods['close_spider'].appendleft(mw.close_spider)
 
     def _process_parallel(self, methodname: str, obj, *args) -> Deferred:
-        return process_parallel(self.methods[methodname], obj, *args)
+        methods = cast(Iterable[Callable], self.methods[methodname])
+        return process_parallel(methods, obj, *args)
 
     def _process_chain(self, methodname: str, obj, *args) -> Deferred:
-        return process_chain(self.methods[methodname], obj, *args)
-
-    def _process_chain_both(self, cb_methodname: str, eb_methodname: str, obj, *args) -> Deferred:
-        return process_chain_both(self.methods[cb_methodname],
-                                  self.methods[eb_methodname], obj, *args)
+        methods = cast(Iterable[Callable], self.methods[methodname])
+        return process_chain(methods, obj, *args)
 
     def open_spider(self, spider: Spider) -> Deferred:
         return self._process_parallel('open_spider', spider)
diff --git a/scrapy/utils/conf.py b/scrapy/utils/conf.py
index 24873f75d..24a6187b9 100644
--- a/scrapy/utils/conf.py
+++ b/scrapy/utils/conf.py
@@ -164,7 +164,7 @@ def feed_process_params_from_cli(settings, output, output_format=None,
             message = (
                 'The -t command line option is deprecated in favor of '
                 'specifying the output format within the output URI. See the '
-                'documentation of the -o and -O options for more information.',
+                'documentation of the -o and -O options for more information.'
             )
             warnings.warn(message, ScrapyDeprecationWarning, stacklevel=2)
             return {output[0]: {'format': output_format}}
diff --git a/scrapy/utils/log.py b/scrapy/utils/log.py
index 0441c0358..78e302d19 100644
--- a/scrapy/utils/log.py
+++ b/scrapy/utils/log.py
@@ -143,7 +143,7 @@ def _get_handler(settings):
     return handler
 
 
-def log_scrapy_info(settings):
+def log_scrapy_info(settings: Settings) -> None:
     logger.info("Scrapy %(version)s started (bot: %(bot)s)",
                 {'version': scrapy.__version__, 'bot': settings['BOT_NAME']})
     versions = [
@@ -152,6 +152,9 @@ def log_scrapy_info(settings):
         if name != "Scrapy"
     ]
     logger.info("Versions: %(versions)s", {'versions': ", ".join(versions)})
+
+
+def log_reactor_info() -> None:
     from twisted.internet import reactor
     logger.debug("Using reactor: %s.%s", reactor.__module__, reactor.__class__.__name__)
     from twisted.internet import asyncioreactor
diff --git a/scrapy/utils/reactor.py b/scrapy/utils/reactor.py
index 6723d9b37..96395543c 100644
--- a/scrapy/utils/reactor.py
+++ b/scrapy/utils/reactor.py
@@ -1,4 +1,5 @@
 import asyncio
+import sys
 from contextlib import suppress
 
 from twisted.internet import asyncioreactor, error
@@ -57,6 +58,10 @@ def install_reactor(reactor_path, event_loop_path=None):
     reactor_class = load_object(reactor_path)
     if reactor_class is asyncioreactor.AsyncioSelectorReactor:
         with suppress(error.ReactorAlreadyInstalledError):
+            if sys.version_info >= (3, 8) and sys.platform == "win32":
+                policy = asyncio.get_event_loop_policy()
+                if not isinstance(policy, asyncio.WindowsSelectorEventLoopPolicy):
+                    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
             if event_loop_path is not None:
                 event_loop_class = load_object(event_loop_path)
                 event_loop = event_loop_class()
diff --git a/scrapy/utils/response.py b/scrapy/utils/response.py
index b3ef7b463..8b109dced 100644
--- a/scrapy/utils/response.py
+++ b/scrapy/utils/response.py
@@ -3,8 +3,9 @@ This module provides some useful functions for working with
 scrapy.http.Response objects
 """
 import os
-import webbrowser
+import re
 import tempfile
+import webbrowser
 from typing import Any, Callable, Iterable, Optional, Tuple, Union
 from weakref import WeakKeyDictionary
 
@@ -80,8 +81,9 @@ def open_in_browser(
     body = response.body
     if isinstance(response, HtmlResponse):
         if b'<base' not in body:
-            repl = f'<head><base href="{response.url}">'
-            body = body.replace(b'<head>', to_bytes(repl))
+            repl = fr'\1<base href="{response.url}">'
+            body = re.sub(b"<!--.*?-->", b"", body, flags=re.DOTALL)
+            body = re.sub(rb"(<head(?:>|\s.*?>))", to_bytes(repl), body)
         ext = '.html'
     elif isinstance(response, TextResponse):
         ext = '.txt'
diff --git a/tests/CrawlerProcess/asyncio_enabled_reactor.py b/tests/CrawlerProcess/asyncio_enabled_reactor.py
index 8568bd8b8..f2a93074b 100644
--- a/tests/CrawlerProcess/asyncio_enabled_reactor.py
+++ b/tests/CrawlerProcess/asyncio_enabled_reactor.py
@@ -1,6 +1,9 @@
 import asyncio
+import sys
 
 from twisted.internet import asyncioreactor
+if sys.version_info >= (3, 8) and sys.platform == "win32":
+    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
 asyncioreactor.install(asyncio.get_event_loop())
 
 import scrapy
diff --git a/tests/CrawlerProcess/twisted_reactor_custom_settings.py b/tests/CrawlerProcess/twisted_reactor_custom_settings.py
new file mode 100644
index 000000000..56304bd23
--- /dev/null
+++ b/tests/CrawlerProcess/twisted_reactor_custom_settings.py
@@ -0,0 +1,14 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+
+
+class AsyncioReactorSpider(scrapy.Spider):
+    name = 'asyncio_reactor'
+    custom_settings = {
+        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+    }
+
+
+process = CrawlerProcess()
+process.crawl(AsyncioReactorSpider)
+process.start()
diff --git a/tests/CrawlerProcess/twisted_reactor_custom_settings_conflict.py b/tests/CrawlerProcess/twisted_reactor_custom_settings_conflict.py
new file mode 100644
index 000000000..3f219098c
--- /dev/null
+++ b/tests/CrawlerProcess/twisted_reactor_custom_settings_conflict.py
@@ -0,0 +1,22 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+
+
+class SelectReactorSpider(scrapy.Spider):
+    name = 'select_reactor'
+    custom_settings = {
+        "TWISTED_REACTOR": "twisted.internet.selectreactor.SelectReactor",
+    }
+
+
+class AsyncioReactorSpider(scrapy.Spider):
+    name = 'asyncio_reactor'
+    custom_settings = {
+        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+    }
+
+
+process = CrawlerProcess()
+process.crawl(SelectReactorSpider)
+process.crawl(AsyncioReactorSpider)
+process.start()
diff --git a/tests/CrawlerProcess/twisted_reactor_custom_settings_same.py b/tests/CrawlerProcess/twisted_reactor_custom_settings_same.py
new file mode 100644
index 000000000..1f5a44010
--- /dev/null
+++ b/tests/CrawlerProcess/twisted_reactor_custom_settings_same.py
@@ -0,0 +1,21 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+
+
+class AsyncioReactorSpider1(scrapy.Spider):
+    name = 'asyncio_reactor1'
+    custom_settings = {
+        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+    }
+
+class AsyncioReactorSpider2(scrapy.Spider):
+    name = 'asyncio_reactor2'
+    custom_settings = {
+        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+    }
+
+
+process = CrawlerProcess()
+process.crawl(AsyncioReactorSpider1)
+process.crawl(AsyncioReactorSpider2)
+process.start()
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 75098a77a..7473b53df 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -674,9 +674,6 @@ class MySpider(scrapy.Spider):
         self.assertIn("start_requests", log)
         self.assertIn("badspider.py", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_asyncio_enabled_true(self):
         log = self.get_log(self.debug_log_spider, args=[
             '-s', 'TWISTED_REACTOR=twisted.internet.asyncioreactor.AsyncioSelectorReactor'
@@ -699,15 +696,15 @@ class MySpider(scrapy.Spider):
         ])
         self.assertIn("Using asyncio event loop: uvloop.Loop", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_custom_asyncio_loop_enabled_false(self):
         log = self.get_log(self.debug_log_spider, args=[
             '-s', 'TWISTED_REACTOR=twisted.internet.asyncioreactor.AsyncioSelectorReactor'
         ])
         import asyncio
-        loop = asyncio.new_event_loop()
+        if sys.platform != 'win32':
+            loop = asyncio.new_event_loop()
+        else:
+            loop = asyncio.SelectorEventLoop()
         self.assertIn(f"Using asyncio event loop: {loop.__module__}.{loop.__class__.__name__}", log)
 
     def test_output(self):
@@ -765,6 +762,7 @@ class MySpider(scrapy.Spider):
         self.assertIn("error: Please use only one of -o/--output and -O/--overwrite-output", log)
 
 
+@skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
 class WindowsRunSpiderCommandTest(RunSpiderCommandTest):
 
     spider_filename = 'myspider.pyw'
@@ -777,35 +775,27 @@ class WindowsRunSpiderCommandTest(RunSpiderCommandTest):
         self.assertIn("start_requests", log)
         self.assertIn("badspider.pyw", log)
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_run_good_spider(self):
         super().test_run_good_spider()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_runspider(self):
         super().test_runspider()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_runspider_dnscache_disabled(self):
         super().test_runspider_dnscache_disabled()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_runspider_log_level(self):
         super().test_runspider_log_level()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_runspider_log_short_names(self):
         super().test_runspider_log_short_names()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_runspider_no_spider_found(self):
         super().test_runspider_no_spider_found()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_output(self):
         super().test_output()
 
-    @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
     def test_overwrite_output(self):
         super().test_overwrite_output()
 
diff --git a/tests/test_crawl.py b/tests/test_crawl.py
index 6c9d8a6ca..cefa79156 100644
--- a/tests/test_crawl.py
+++ b/tests/test_crawl.py
@@ -278,6 +278,28 @@ with multiples lines
         self.assertEqual(s['engine.spider.name'], crawler.spider.name)
         self.assertEqual(s['len(engine.scraper.slot.active)'], 1)
 
+    @defer.inlineCallbacks
+    def test_format_engine_status(self):
+        from scrapy.utils.engine import format_engine_status
+        est = []
+
+        def cb(response):
+            est.append(format_engine_status(crawler.engine))
+
+        crawler = self.runner.create_crawler(SingleRequestSpider)
+        yield crawler.crawl(seed=self.mockserver.url('/'), callback_func=cb, mockserver=self.mockserver)
+        self.assertEqual(len(est), 1, est)
+        est = est[0].split("\n")[2:-2]  # remove header & footer
+        # convert to dict
+        est = [x.split(":") for x in est]
+        est = [x for sublist in est for x in sublist]  # flatten
+        est = [x.lstrip().rstrip() for x in est]
+        it = iter(est)
+        s = dict(zip(it, it))
+
+        self.assertEqual(s['engine.spider.name'], crawler.spider.name)
+        self.assertEqual(s['len(engine.scraper.slot.active)'], '1')
+
     @defer.inlineCallbacks
     def test_graceful_crawl_error_handling(self):
         """
diff --git a/tests/test_crawler.py b/tests/test_crawler.py
index be067155e..8f6227109 100644
--- a/tests/test_crawler.py
+++ b/tests/test_crawler.py
@@ -4,10 +4,8 @@ import platform
 import subprocess
 import sys
 import warnings
-from unittest import skipIf
 
 from pytest import raises, mark
-from testfixtures import LogCapture
 from twisted import version as twisted_version
 from twisted.internet import defer
 from twisted.python.versions import Version
@@ -271,6 +269,7 @@ class CrawlerRunnerHasSpider(unittest.TestCase):
 
         self.assertEqual(runner.bootstrap_failed, True)
 
+    @defer.inlineCallbacks
     def test_crawler_runner_asyncio_enabled_true(self):
         if self.reactor_pytest == 'asyncio':
             CrawlerRunner(settings={
@@ -279,35 +278,10 @@ class CrawlerRunnerHasSpider(unittest.TestCase):
         else:
             msg = r"The installed reactor \(.*?\) does not match the requested one \(.*?\)"
             with self.assertRaisesRegex(Exception, msg):
-                CrawlerRunner(settings={
-                    "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
-                })
-
-    @defer.inlineCallbacks
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
-    def test_crawler_process_asyncio_enabled_true(self):
-        with LogCapture(level=logging.DEBUG) as log:
-            if self.reactor_pytest == 'asyncio':
-                runner = CrawlerProcess(settings={
+                runner = CrawlerRunner(settings={
                     "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
                 })
                 yield runner.crawl(NoRequestsSpider)
-                self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", str(log))
-            else:
-                msg = r"The installed reactor \(.*?\) does not match the requested one \(.*?\)"
-                with self.assertRaisesRegex(Exception, msg):
-                    runner = CrawlerProcess(settings={
-                        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
-                    })
-
-    @defer.inlineCallbacks
-    def test_crawler_process_asyncio_enabled_false(self):
-        runner = CrawlerProcess(settings={"TWISTED_REACTOR": None})
-        with LogCapture(level=logging.DEBUG) as log:
-            yield runner.crawl(NoRequestsSpider)
-            self.assertNotIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", str(log))
 
 
 class ScriptRunnerMixin:
@@ -328,17 +302,11 @@ class CrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
         self.assertIn('Spider closed (finished)', log)
         self.assertNotIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_asyncio_enabled_no_reactor(self):
         log = self.run_script('asyncio_enabled_no_reactor.py')
         self.assertIn('Spider closed (finished)', log)
         self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_asyncio_enabled_reactor(self):
         log = self.run_script('asyncio_enabled_reactor.py')
         self.assertIn('Spider closed (finished)', log)
@@ -377,14 +345,26 @@ class CrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.pollreactor.PollReactor", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_reactor_asyncio(self):
         log = self.run_script("twisted_reactor_asyncio.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
+    def test_reactor_asyncio_custom_settings(self):
+        log = self.run_script("twisted_reactor_custom_settings.py")
+        self.assertIn("Spider closed (finished)", log)
+        self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
+
+    def test_reactor_asyncio_custom_settings_same(self):
+        log = self.run_script("twisted_reactor_custom_settings_same.py")
+        self.assertIn("Spider closed (finished)", log)
+        self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
+
+    def test_reactor_asyncio_custom_settings_conflict(self):
+        log = self.run_script("twisted_reactor_custom_settings_conflict.py")
+        self.assertIn("Using reactor: twisted.internet.selectreactor.SelectReactor", log)
+        self.assertIn("(twisted.internet.selectreactor.SelectReactor) does not match the requested one", log)
+
     @mark.skipif(sys.implementation.name == 'pypy', reason='uvloop does not support pypy properly')
     @mark.skipif(platform.system() == 'Windows', reason='uvloop does not support Windows')
     @mark.skipif(twisted_version == Version('twisted', 21, 2, 0), reason='https://twistedmatrix.com/trac/ticket/10106')
@@ -404,9 +384,6 @@ class CrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
         self.assertIn("Using asyncio event loop: uvloop.Loop", log)
         self.assertIn("async pipeline opened!", log)
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_default_loop_asyncio_deferred_signal(self):
         log = self.run_script("asyncio_deferred_signal.py")
         self.assertIn("Spider closed (finished)", log)
diff --git a/tests/test_downloader_handlers.py b/tests/test_downloader_handlers.py
index 9c11820e5..a1ea4c679 100644
--- a/tests/test_downloader_handlers.py
+++ b/tests/test_downloader_handlers.py
@@ -1,6 +1,7 @@
 import contextlib
 import os
 import shutil
+import sys
 import tempfile
 from typing import Optional, Type
 from unittest import mock
@@ -287,6 +288,12 @@ class HttpTestCase(unittest.TestCase):
 
     @defer.inlineCallbacks
     def test_timeout_download_from_spider_nodata_rcvd(self):
+        if self.reactor_pytest == "asyncio" and sys.platform == "win32":
+            # https://twistedmatrix.com/trac/ticket/10279
+            raise unittest.SkipTest(
+                "This test produces DirtyReactorAggregateError on Windows with asyncio"
+            )
+
         # client connects but no data is received
         spider = Spider('foo')
         meta = {'download_timeout': 0.5}
@@ -296,6 +303,11 @@ class HttpTestCase(unittest.TestCase):
 
     @defer.inlineCallbacks
     def test_timeout_download_from_spider_server_hangs(self):
+        if self.reactor_pytest == "asyncio" and sys.platform == "win32":
+            # https://twistedmatrix.com/trac/ticket/10279
+            raise unittest.SkipTest(
+                "This test produces DirtyReactorAggregateError on Windows with asyncio"
+            )
         # client connects, server send headers and some body bytes but hangs
         spider = Spider('foo')
         meta = {'download_timeout': 0.5}
@@ -1055,6 +1067,10 @@ class BaseFTPTestCase(unittest.TestCase):
 class FTPTestCase(BaseFTPTestCase):
 
     def test_invalid_credentials(self):
+        if self.reactor_pytest == "asyncio" and sys.platform == "win32":
+            raise unittest.SkipTest(
+                "This test produces DirtyReactorAggregateError on Windows with asyncio"
+            )
         from twisted.protocols.ftp import ConnectionLost
 
         meta = dict(self.req_meta)
diff --git a/tests/test_request_left.py b/tests/test_request_left.py
index 373b2e49c..4d4483881 100644
--- a/tests/test_request_left.py
+++ b/tests/test_request_left.py
@@ -22,7 +22,7 @@ class SignalCatcherSpider(Spider):
         return spider
 
     def on_request_left(self, request, spider):
-        self.caught_times = self.caught_times + 1
+        self.caught_times += 1
 
 
 class TestCatching(TestCase):
diff --git a/tests/test_utils_asyncio.py b/tests/test_utils_asyncio.py
index a2114bd18..295323e4d 100644
--- a/tests/test_utils_asyncio.py
+++ b/tests/test_utils_asyncio.py
@@ -1,6 +1,4 @@
-import platform
-import sys
-from unittest import skipIf, TestCase
+from unittest import TestCase
 
 from pytest import mark
 
@@ -14,9 +12,6 @@ class AsyncioTest(TestCase):
         # the result should depend only on the pytest --reactor argument
         self.assertEqual(is_asyncio_reactor_installed(), self.reactor_pytest == 'asyncio')
 
-    # https://twistedmatrix.com/trac/ticket/9766
-    @skipIf(platform.system() == 'Windows' and sys.version_info >= (3, 8),
-            "the asyncio reactor is broken on Windows when running Python ≥ 3.8")
     def test_install_asyncio_reactor(self):
         # this should do nothing
         install_reactor("twisted.internet.asyncioreactor.AsyncioSelectorReactor")
diff --git a/tests/test_utils_response.py b/tests/test_utils_response.py
index d6f4c0bb5..0a09f6109 100644
--- a/tests/test_utils_response.py
+++ b/tests/test_utils_response.py
@@ -83,3 +83,56 @@ class ResponseUtilsTest(unittest.TestCase):
         self.assertEqual(response_status_message(200), '200 OK')
         self.assertEqual(response_status_message(404), '404 Not Found')
         self.assertEqual(response_status_message(573), "573 Unknown Status")
+
+    def test_inject_base_url(self):
+        url = "http://www.example.com"
+
+        def check_base_url(burl):
+            path = urlparse(burl).path
+            if not os.path.exists(path):
+                path = burl.replace('file://', '')
+            with open(path, "rb") as f:
+                bbody = f.read()
+            self.assertEqual(bbody.count(b'<base href="' + to_bytes(url) + b'">'), 1)
+            return True
+
+        r1 = HtmlResponse(url, body=b"""
+        <html>
+            <head><title>Dummy</title></head>
+            <body><p>Hello world.</p></body>
+        </html>""")
+        r2 = HtmlResponse(url, body=b"""
+        <html>
+            <head id="foo"><title>Dummy</title></head>
+            <body>Hello world.</body>
+        </html>""")
+        r3 = HtmlResponse(url, body=b"""
+        <html>
+            <head><title>Dummy</title></head>
+            <body>
+                <header>Hello header</header>
+                <p>Hello world.</p>
+            </body>
+        </html>""")
+        r4 = HtmlResponse(url, body=b"""
+        <html>
+            <!-- <head>Dummy comment</head> -->
+            <head><title>Dummy</title></head>
+            <body><p>Hello world.</p></body>
+        </html>""")
+        r5 = HtmlResponse(url, body=b"""
+        <html>
+            <!--[if IE]>
+            <head><title>IE head</title></head>
+            <![endif]-->
+            <!--[if !IE]>-->
+            <head><title>Standard head</title></head>
+            <!--<![endif]-->
+            <body><p>Hello world.</p></body>
+        </html>""")
+
+        assert open_in_browser(r1, _openfunc=check_base_url), "Inject base url"
+        assert open_in_browser(r2, _openfunc=check_base_url), "Inject base url with argumented head"
+        assert open_in_browser(r3, _openfunc=check_base_url), "Inject unique base url with misleading tag"
+        assert open_in_browser(r4, _openfunc=check_base_url), "Inject unique base url with misleading comment"
+        assert open_in_browser(r5, _openfunc=check_base_url), "Inject unique base url with conditional comment"
diff --git a/tox.ini b/tox.ini
index 021dd9988..2031a2d92 100644
--- a/tox.ini
+++ b/tox.ini
@@ -29,7 +29,7 @@ passenv =
 #allow tox virtualenv to upgrade pip/wheel/setuptools
 download = true
 commands =
-    py.test --cov=scrapy --cov-report=xml --cov-report= {posargs:--durations=10 docs scrapy tests}
+    pytest --cov=scrapy --cov-report=xml --cov-report= {posargs:--durations=10 docs scrapy tests}
 install_command =
     pip install -U -ctests/upper-constraints.txt {opts} {packages}
 
@@ -60,13 +60,13 @@ deps =
     pytest-flake8
     flake8==3.9.2  # https://github.com/tholo/pytest-flake8/issues/81
 commands =
-    py.test --flake8 {posargs:docs scrapy tests}
+    pytest --flake8 {posargs:docs scrapy tests}
 
 [testenv:pylint]
 basepython = python3
 deps =
     {[testenv:extra-deps]deps}
-    pylint
+    pylint==2.12.1
 commands =
     pylint conftest.py docs extras scrapy setup.py tests
 
@@ -142,7 +142,7 @@ setenv =
 [testenv:pypy3]
 basepython = pypy3
 commands =
-    py.test {posargs:--durations=10 docs scrapy tests}
+    pytest {posargs:--durations=10 docs scrapy tests}
 
 [testenv:pypy3-pinned]
 basepython = {[testenv:pypy3]basepython}