html5lib
diff --git a/.appveyor.yml b/.appveyor.yml
@@ -9,14 +9,6 @@ environment:
     - TOXENV: py35-optional
     - TOXENV: py36-base
     - TOXENV: py36-optional
-    - TOXENV: py37-base
-    - TOXENV: py37-optional
-    - TOXENV: py38-base
-    - TOXENV: py38-optional
-    - TOXENV: py39-base
-    - TOXENV: py39-optional
-    - TOXENV: py310-base
-    - TOXENV: py310-optional
 
 install:
   - git submodule update --init --recursive

diff --git a/.github/workflows/python-tox.yml b/.github/workflows/python-tox.yml
@@ -4,21 +4,62 @@ jobs:
     # Prevent duplicate builds for 'internal' pull requests on existing commits
     # Credit: https://github.community/t/duplicate-checks-on-push-and-pull-request-simultaneous-event/18012
     if: github.event.push || github.event.pull_request.head.repo.full_name != github.repository
-    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        python: ["2.7", "3.7", "3.8", "3.9", "3.10", "3.11", "pypy-2.7", "pypy-3.8"]
+        # 2.7, 3.5, and 3.6 run on Windows via AppVeyor
+        python: ["3.7", "3.8", "3.9", "3.10", "3.11"]
+        os: [ubuntu-latest, windows-latest]
+        deps: [base, optional]
+        include:
+          - python: "pypy-2.7"
+            os: ubuntu-latest
+            deps: base
+          - python: "pypy-3.8"
+            os: ubuntu-latest
+            deps: base
+          - python: "2.7"
+            os: ubuntu-latest
+            deps: oldest
+          - python: "3.7"
+            os: ubuntu-latest
+            deps: oldest
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
         with:
           submodules: true
-      - uses: actions/setup-python@v4
+      - if: ${{ matrix.deps == 'base' }}
+        uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python }}
           cache: pip
-          cache-dependency-path: "requirements*.txt"
+          cache-dependency-path: |
+            requirements.txt
+            requirements-test.txt
+      - if: ${{ matrix.deps == 'optional' }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+          cache: pip
+          cache-dependency-path: |
+            requirements.txt
+            requirements-optional.txt
+            requirements-test.txt
+      - if: ${{ matrix.deps == 'oldest' }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+          cache: pip
+          cache-dependency-path: |
+            requirements-oldest.txt
+      - if: ${{ matrix.os == 'windows-latest' }}
+        name: Determine environment name for Tox (PowerShell)
+        run: python toxver.py ${{ matrix.python }} ${{ matrix.deps }} >> $env:GITHUB_ENV
+      - if: ${{ matrix.os == 'ubuntu-latest' }}
+        name: Determine environment name for Tox (Bash)
+        run: python toxver.py ${{ matrix.python }} ${{ matrix.deps }} >> $GITHUB_ENV
       - run: pip install tox
-      - run: tox -e py
+      - run: tox
       - if: ${{ always() }}
         run: python debug-info.py
diff --git a/requirements-install.sh b/requirements-install.sh
diff --git a/requirements-oldest.txt b/requirements-oldest.txt
@@ -0,0 +1,29 @@
+# This allows us to install the actually oldest supported dependencies and test whether that works.
+
+# requirements.txt
+six==1.9
+webencodings==0.5.1
+
+# requirements-optional.txt
+genshi==0.7.1 ; python_version < '3.8'
+genshi==0.7.6 ; python_version >= '3.8'
+chardet==2.2.1
+# this should be 3.4.0 but there are no Linux
+# binary wheels for older releases 
+lxml==3.8.0 ; python_version < '3.7'
+# minimums for 3.x are actually different:
+# - 3.7 is actually 4.1.1
+# - 3.8 is actually 4.3.5
+# - 3.9-3.10 is actually 4.5.2
+# - 3.11 is actually 4.9.0
+lxml==4.9.0 ; python_version >= '3.7'
+
+# requirements-test.txt
+flake8==3.9.2 ; python_version < '3.6'
+flake8==5.0.4; python_version >= '3.6'
+pytest==4.6.10 ; python_version < '3'
+pytest==5.4.2 ; python_version >= '3'
+coverage==5.1
+pytest-expect==1.1.0
+mock==3.0.5 ; python_version < '3.6'
+mock==4.0.2 ; python_version >= '3.6'
diff --git a/requirements-optional.txt b/requirements-optional.txt
@@ -2,12 +2,12 @@
 
 # We support a Genshi treewalker that can be used to serialize Genshi
 # streams.
-genshi
+genshi>=0.7.1
 
 # chardet can be used as a fallback in case we are unable to determine
 # the encoding of a document.
-chardet>=2.2
+chardet>=2.2.1
 
 # lxml is supported with its own treebuilder ("lxml") and otherwise
 # uses the standard ElementTree support
-lxml ; platform_python_implementation == 'CPython'
+lxml>=3.4.0 ; platform_python_implementation == 'CPython'
diff --git a/requirements-test.txt b/requirements-test.txt
@@ -1,7 +1,7 @@
 -r requirements.txt
 
-tox>=3.15.1,<4
-flake8>=3.8.1,<6
+flake8==3.9.2 ; python_version < '3.6'
+flake8>=5.0.4; python_version >= '3.6'
 pytest>=4.6.10,<5 ; python_version < '3'
 pytest>=5.4.2,<7 ; python_version >= '3'
 coverage>=5.1,<6

diff --git a/setup.py b/setup.py
@@ -108,23 +108,23 @@ def default_environment():
       packages=find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
       install_requires=[
           'six>=1.9',
-          'webencodings',
+          'webencodings>=0.5.1',
       ],
       python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*",
       extras_require={
           # A conditional extra will only install these items when the extra is
           # requested and the condition matches.
-          "lxml:platform_python_implementation == 'CPython'": ["lxml"],
+          "lxml:platform_python_implementation == 'CPython'": ["lxml>=3.4.0"],
 
           # Standard extras, will be installed when the extra is requested.
-          "genshi": ["genshi"],
-          "chardet": ["chardet>=2.2"],
+          "genshi": ["genshi>=0.7.1"],
+          "chardet": ["chardet>=2.2.1"],
 
           # The all extra combines a standard extra which will be used anytime
           # the all extra is requested, and it extends it with a conditional
           # extra that will be installed whenever the condition matches and the
           # all extra is requested.
-          "all": ["genshi", "chardet>=2.2"],
-          "all:platform_python_implementation == 'CPython'": ["lxml"],
+          "all": ["genshi>=0.7.1", "chardet>=2.2.1"],
+          "all:platform_python_implementation == 'CPython'": ["lxml>=3.4.0"],
       },
       )
diff --git a/tox.ini b/tox.ini
@@ -1,17 +1,18 @@
 [tox]
-envlist = py{27,35,36,37,38,39,310,311,py,py3}-{base,six19,optional}
+envlist = py{27,35,36,37,38,39,310,311,py,py3}-{base,optional,oldest}
 
 [testenv]
 deps =
+  base: -r{toxinidir}/requirements-test.txt
+  optional: -r{toxinidir}/requirements-test.txt
   optional: -r{toxinidir}/requirements-optional.txt
-  -r{toxinidir}/requirements-test.txt
+  oldest: -r{toxinidir}/requirements-oldest.txt
   doc: Sphinx
 
 passenv =
   PYTEST_COMMAND  # this is maintained so one can, e.g., PYTEST_COMMAND="coverage run -m pytest"
   COVERAGE_RUN_OPTIONS
 commands =
-  six19: pip install six==1.9
   {env:PYTEST_COMMAND:{envbindir}/pytest} {posargs}
   flake8 {toxinidir}
 

diff --git a/toxver.py b/toxver.py
@@ -0,0 +1,57 @@
+#!/usr/bin/env python
+
+"""
+usage: toxver.py [python-version] [deps]
+
+Returns a Tox environment name given a GHA matrix Python version and dependencies.
+Many GHA configurations do this with inline Bash scripts but we want our solution
+to be cross-platform and work on Windows workers, too.
+
+Examples:
+
+    $ toxver.py pypy-3.8 base
+    TOXENV=pypy3-base
+
+    $ toxver.py 2.7 oldest
+    TOXENV=py27-oldest
+
+    $ toxver.py ~3.12.0-0 optional
+    TOXENV=py312-optional
+
+"""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import sys
+
+
+def main(argv):
+    if len(argv) != 3:
+        print(__doc__.strip(), file=sys.stderr)
+        return 1
+
+    deps = argv[2]
+
+    if argv[1].startswith("pypy-2"):
+        print("TOXENV=pypy-" + deps)
+        return 0
+
+    if argv[1].startswith("pypy-3"):
+        print("TOXENV=pypy3-" + deps)
+        return 0
+
+    if argv[1].startswith("~"):
+        ver = argv[1][1:5]
+    else:
+        ver = argv[1]
+
+    ver = ver.replace(".", "")
+    print("TOXENV=py" + ver + "-" + deps)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))