diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 9566d06d..bce81ade 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -206,6 +206,20 @@ jobs:
       - name: Lint
         run: tox -c pyproject.toml -e flake8,mypy,isort
 
+  htmlvalidator:
+    name: HTML Validator
+    runs-on: ubuntu-latest
+    needs: test-ubuntu-python-newest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.14'
+      - uses: Cyb3r-Jak3/html5validator-action@v7.2.0
+        with:
+          root: 'radicale/web/internal_data/'
+          css: true
+
   integ-test:
     timeout-minutes: 60
     runs-on: ubuntu-latest
diff --git a/pyproject.toml b/pyproject.toml
index fbf8b38b..010f5f29 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -44,7 +44,7 @@ test = ["pytest>=7", "waitress", "bcrypt", "argon2-cffi"]
 bcrypt = ["bcrypt"]
 argon2 = ["argon2-cffi"]
 ldap = ["ldap3"]
-dev = ["flake8", "isort", "mypy", "pytest", "pytest-playwright"]
+dev = ["flake8", "isort", "mypy", "pytest", "pytest-playwright", "html5validator"]
 
 [project.scripts]
 radicale = "radicale.__main__:run"
@@ -55,7 +55,7 @@ build-backend = "setuptools.build_meta"
 
 [tool.tox]
 min_version = "4.0"
-envlist = ["py", "flake8", "isort", "mypy", "integ_test"]
+envlist = ["py", "flake8", "isort", "mypy", "integ_test", "html5validator"]
 
 [tool.tox.env.py]
 extras = ["test"]
@@ -87,6 +87,11 @@ setenv = { PLAYWRIGHT_BROWSERS_PATH = "{toxworkdir}/.playwright" }
 commands_pre = [["playwright", "install", "chromium"]]
 commands = [["pytest", "integ_tests"]]
 
+[tool.tox.env.html5validator]
+deps = ["html5validator"]
+commands = [["html5validator", "--root", "radicale/web/internal_data/", "--also-check-css"]]
+
+
 [tool.pytest.ini_options]
 norecursedirs = ["integ_tests"]