diff --git a/.oxfmtrc.json b/.oxfmtrc.json
new file mode 100644
index 00000000..9da89736
--- /dev/null
+++ b/.oxfmtrc.json
@@ -0,0 +1,52 @@
+{
+  // oxfmt formats the JS, CSS, JSON and YAML; `just fmt-js` runs it, and
+  // `just fmt-check` checks. See "Formatting" in CONTRIBUTING.md.
+  // Comments here must sit on lines of their own: scripts/fmt_templates.py
+  // reads this file too.
+  "printWidth": 120,
+  "tabWidth": 4,
+  "singleQuote": true,
+  "trailingComma": "none",
+  "arrowParens": "avoid",
+  "overrides": [
+    {
+      "files": ["*.css"],
+      "options": { "singleQuote": false }
+    },
+    {
+      "files": ["*.yml", "*.yaml"],
+      "options": { "tabWidth": 2, "singleQuote": false }
+    },
+    {
+      "files": ["*.json", "*.jsonc"],
+      "options": { "tabWidth": 2 }
+    },
+    {
+      "files": ["**/.vscode/settings.json"],
+      "options": { "tabWidth": 4 }
+    }
+  ],
+  "ignorePatterns": [
+    // Markdown and TOML stay as written (oxfmt collapses TOML arrays that
+    // `uv add` expands again).
+    "**/*.md",
+    "**/*.toml",
+    // Data, generated output and plans.
+    "test/test_data/**",
+    "test/__snapshots__/**",
+    "dev-docs/messages/**",
+    "site/**",
+    "work/**",
+    "scripts/style_guide_output/**",
+    // Read line by line by scripts/generate_minimal_pygments_css.py, which
+    // generates the second one.
+    "claude_code_log/html/templates/components/pygments_styles.css",
+    "claude_code_log/html/templates/components/minimal/pygments_dark.css",
+    // Jinja inside: formatted by scripts/fmt_templates.py instead.
+    "claude_code_log/html/templates/components/minimal/chrome.css",
+    "claude_code_log/html/templates/components/minimal/layout.css",
+    "claude_code_log/html/templates/components/minimal/minimal.js",
+    "claude_code_log/html/templates/components/minimal/pages.js",
+    "claude_code_log/html/templates/components/minimal/theme_init.js"
+  ]
+}
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 9c36a039..51557172 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -235,6 +235,10 @@ uv run ty check
 
 # Lint the templates' JavaScript (installs the optional `js` group)
 just lint-js
+
+# Format JS, CSS, JSON and YAML, or only check (same group)
+just fmt-js
+just fmt-check
 ```
 
 The HTML pages carry a fair amount of JavaScript: the files under
@@ -268,6 +272,28 @@ generated — don't edit it by hand. After a Pygments upgrade or a change to
 `uv run python scripts/generate_minimal_pygments_css.py`; a unit test fails
 while the committed file differs from the script's output.
 
+### Formatting JS, CSS, JSON and YAML
+
+[oxfmt](https://oxc.rs/docs/guide/usage/formatter.html), oxlint's sibling,
+formats the JavaScript, CSS, JSON and YAML; it comes from the same `js`
+group, pinned to an exact version because its output changes between
+releases. `.oxfmtrc.json` holds the options, and its `ignorePatterns`
+leave out, each with its reason: Markdown and TOML, data and generated
+files, and the two Pygments sheets, which
+`scripts/generate_minimal_pygments_css.py` reads line by line.
+
+A few component files hold Jinja (an `{% include %}`, a `{# comment #}`),
+which oxfmt cannot parse. `just fmt-js` formats them through
+`scripts/fmt_templates.py`, which masks each Jinja construct as a
+`/* jinja:N */` comment, formats the masked copy, and puts the constructs
+back verbatim. Each construct must sit on lines of its own; the script
+refuses anything else. Such files are found by their content, but they must
+also be listed in `ignorePatterns`, which a unit test checks.
+
+When upgrading oxfmt, look at what a run *covers* as well as whether it
+passes: a release can start formatting a new language (Markdown arrived
+after 0.69), and then a bare run rewrites files nobody chose to format.
+
 ### Whitespace
 
 An [`.editorconfig`](.editorconfig) at the repo root defines the baseline —
diff --git a/justfile b/justfile
index 6f9bcbf0..e21d760e 100644
--- a/justfile
+++ b/justfile
@@ -92,6 +92,22 @@ ty:
 lint-js *ARGS:
     uv run --group js python scripts/lint_js.py {{ ARGS }}
 
+# Format the JS, CSS, JSON and YAML with oxfmt (installs the optional `js`
+# group); the template components holding Jinja go through
+# scripts/fmt_templates.py. Configured in .oxfmtrc.json.
+fmt-js:
+    uv run --group js oxfmt
+    uv run --group js python scripts/fmt_templates.py
+
+# Check that formatting, writing nothing. Both checks run, so every file
+# that needs formatting is listed.
+fmt-check:
+    #!/usr/bin/env bash
+    status=0
+    uv run --group js oxfmt --check || status=1
+    uv run --group js python scripts/fmt_templates.py --check || status=1
+    exit $status
+
 # Fail-fast order: format and lint take seconds, ty is faster than pyright, the full test suite runs last
 ci: format lint lint-js ty typecheck test-all
 
diff --git a/pyproject.toml b/pyproject.toml
index 3be25b61..caa215e1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -186,6 +186,7 @@ dev = [
 # official oxc-project release (packaging: github.com/dhruvkb/oxc-py), so
 # it is pinned exactly and bumped deliberately.
 js = [
+    "oxfmt==0.69.0",
     "oxlint==1.84.0",
 ]
 
diff --git a/scripts/fmt_templates.py b/scripts/fmt_templates.py
new file mode 100644
index 00000000..d8a462f4
--- /dev/null
+++ b/scripts/fmt_templates.py
@@ -0,0 +1,176 @@
+#!/usr/bin/env python3
+"""Format the template components that carry Jinja, with oxfmt.
+
+oxfmt formats the rest of the JS and CSS directly (``just fmt-js``), but a
+few files under ``html/templates/components/`` hold a Jinja ``{% include %}``
+or ``{# comment #}``, which no JS or CSS parser accepts. ``.oxfmtrc.json``
+ignores them, and this script formats them instead:
+
+1. **mask** — each Jinja construct becomes a ``/* jinja:N */`` placeholder,
+   a comment in both JS and CSS;
+2. **format** — oxfmt formats the masked copy in a staging directory, with
+   the repo's configuration;
+3. **unmask** — each placeholder is replaced by its construct, verbatim and
+   at its original column.
+
+A construct must sit on lines of its own; anything else is refused rather
+than guessed at. The files are found by content, so a new include is picked
+up without a list to update, but it must also be added to ``ignorePatterns``
+(the plain oxfmt run cannot parse it), which this script checks.
+
+``--check`` writes nothing, lists the files that would change and exits 1
+if there are any. It cannot use ``oxfmt --check`` on the masked copy:
+oxfmt indents a placeholder that the file itself keeps at column 0.
+
+oxfmt comes from the optional ``js`` dependency group
+(``uv sync --group js``); ``just fmt-js`` and ``just fmt-check`` run this
+script with it.
+"""
+
+from __future__ import annotations
+
+import json
+import re
+import shutil
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+from typing import Any
+
+REPO = Path(__file__).resolve().parent.parent
+COMPONENTS = REPO / "claude_code_log" / "html" / "templates" / "components"
+CONFIG = REPO / ".oxfmtrc.json"
+
+JINJA = re.compile(r"\{%.*?%\}|\{#.*?#\}|\{\{.*?\}\}", re.DOTALL)
+PLACEHOLDER = re.compile(r"^[ \t]*/\* jinja:(\d+) \*/[ \t]*$", re.MULTILINE)
+
+
+class MaskError(ValueError):
+    pass
+
+
+def mask(text: str) -> tuple[str, list[str]]:
+    """Replace each Jinja construct's lines by a placeholder comment line.
+
+    Returns the masked text and the replaced lines (without their final
+    newline), indexed by placeholder number.
+    """
+    blocks: list[str] = []
+    parts: list[str] = []
+    pos = 0
+    for match in JINJA.finditer(text):
+        start = text.rfind("\n", 0, match.start()) + 1
+        end = text.find("\n", match.end())
+        end = len(text) if end < 0 else end
+        before, after = text[start : match.start()], text[match.end() : end]
+        if before.strip() or after.strip() or start < pos:
+            raise MaskError(
+                f"Jinja must sit on lines of its own: {match.group()[:60]!r}"
+            )
+        parts += [text[pos:start], f"/* jinja:{len(blocks)} */"]
+        blocks.append(text[start:end])
+        pos = end
+    parts.append(text[pos:])
+    return "".join(parts), blocks
+
+
+def unmask(text: str, blocks: list[str]) -> str:
+    """Put back the lines :func:`mask` replaced, whatever their indent now."""
+    seen: list[int] = []
+
+    def restore(match: re.Match[str]) -> str:
+        seen.append(int(match.group(1)))
+        return blocks[seen[-1]]
+
+    text = PLACEHOLDER.sub(restore, text)
+    if sorted(seen) != list(range(len(blocks))):
+        raise MaskError(f"placeholders lost or duplicated: {seen}")
+    return text
+
+
+def read_config() -> dict[str, Any]:
+    """Parse ``.oxfmtrc.json``, whose comments sit on lines of their own."""
+    text = CONFIG.read_text(encoding="utf-8")
+    return json.loads(re.sub(r"^\s*//.*$", "", text, flags=re.MULTILINE))
+
+
+def jinja_components() -> list[str]:
+    """The component JS/CSS files that hold Jinja, as repo-relative paths."""
+    found: list[str] = []
+    for path in sorted([*COMPONENTS.rglob("*.js"), *COMPONENTS.rglob("*.css")]):
+        if JINJA.search(path.read_text(encoding="utf-8")):
+            found.append(path.relative_to(REPO).as_posix())
+    return found
+
+
+def _read(path: Path) -> str:
+    return path.read_bytes().decode("utf-8")
+
+
+def _write(path: Path, text: str) -> None:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_bytes(text.encode("utf-8"))
+
+
+def main(argv: list[str]) -> int:
+    check = "--check" in argv
+    oxfmt = shutil.which("oxfmt")
+    if oxfmt is None:
+        print(
+            "oxfmt not found: install the optional `js` group with"
+            " `uv sync --group js`, or run `just fmt-js`.",
+            file=sys.stderr,
+        )
+        return 2
+
+    config = read_config()
+    files = jinja_components()
+    unlisted = [f for f in files if f not in config.get("ignorePatterns", [])]
+    if unlisted:
+        print(
+            "fmt-templates: add these to ignorePatterns in .oxfmtrc.json"
+            " (they hold Jinja, which plain oxfmt cannot parse):",
+            *unlisted,
+            sep="\n  ",
+            file=sys.stderr,
+        )
+        return 1
+
+    # The staged copy is formatted with the repo's options, but not its
+    # ignorePatterns, which name exactly these files.
+    del config["ignorePatterns"]
+    with tempfile.TemporaryDirectory(prefix="fmt-templates-") as tmp:
+        stage = Path(tmp)
+        _write(stage / "oxfmtrc.json", json.dumps(config))
+        blocks: dict[str, list[str]] = {}
+        for f in files:
+            masked, blocks[f] = mask(_read(REPO / f))
+            _write(stage / f, masked)
+        result = subprocess.run(
+            [oxfmt, "-c", "oxfmtrc.json", "--write", *files],
+            cwd=stage,
+            capture_output=True,
+            text=True,
+        )
+        if result.returncode:
+            print(result.stdout + result.stderr, file=sys.stderr)
+            return result.returncode
+
+        changed: list[str] = []
+        for f in files:
+            formatted = unmask(_read(stage / f), blocks[f])
+            if formatted != _read(REPO / f):
+                changed.append(f)
+                if not check:
+                    _write(REPO / f, formatted)
+
+    verb = "would reformat" if check else "reformatted"
+    for f in changed:
+        print(f"fmt-templates: {verb} {f}")
+    print(f"fmt-templates: {len(files)} files with Jinja, {len(changed)} {verb}")
+    return 1 if check and changed else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))
diff --git a/test/test_fmt_templates.py b/test/test_fmt_templates.py
new file mode 100644
index 00000000..d1ae6fae
--- /dev/null
+++ b/test/test_fmt_templates.py
@@ -0,0 +1,88 @@
+"""Tests for scripts/fmt_templates.py, which formats the template components
+holding Jinja by masking it (`just fmt-js`, `just fmt-check`). Only the
+masking is tested here: oxfmt is in the optional `js` group."""
+
+import importlib.util
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+_SCRIPT = Path(__file__).parent.parent / "scripts" / "fmt_templates.py"
+
+
+def _load_script() -> Any:
+    spec = importlib.util.spec_from_file_location("fmt_templates", _SCRIPT)
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+fmt_templates = _load_script()
+
+_SOURCE = """{# A header comment
+   on two lines. -#}
+        (function () {
+            'use strict';
+{% include 'components/minimal/scheme.js' %}
+            run();
+        })();
+"""
+
+
+def test_mask_replaces_each_construct_by_one_placeholder_line():
+    masked, blocks = fmt_templates.mask(_SOURCE)
+    assert masked.splitlines()[0] == "/* jinja:0 */"
+    assert masked.splitlines()[3] == "/* jinja:1 */"
+    assert "{" + "%" not in masked and "{" + "#" not in masked
+    assert blocks == [
+        "{# A header comment\n   on two lines. -#}",
+        "{% include 'components/minimal/scheme.js' %}",
+    ]
+
+
+def test_unmask_restores_verbatim_whatever_the_new_indent():
+    masked, blocks = fmt_templates.mask(_SOURCE)
+    # What a formatter does to the masked copy: dedent, re-indent the
+    # placeholder inside the function.
+    formatted = masked.replace("        ", "").replace(
+        "/* jinja:1 */", "    /* jinja:1 */"
+    )
+    restored = fmt_templates.unmask(formatted, blocks)
+    assert "\n{% include 'components/minimal/scheme.js' %}\n" in restored
+    assert restored.startswith(blocks[0] + "\n")
+    assert fmt_templates.unmask(masked, blocks) == _SOURCE
+
+
+@pytest.mark.parametrize(
+    "source",
+    [
+        "const theme = '{{ theme }}';\n",
+        "{% if a %}{% include 'x.js' %}\n",
+        "{% if a %}\n.x { color: red; } {# trailing #}\n",
+    ],
+)
+def test_mask_refuses_jinja_sharing_a_line(source: str):
+    with pytest.raises(fmt_templates.MaskError):
+        fmt_templates.mask(source)
+
+
+def test_unmask_refuses_a_lost_or_duplicated_placeholder():
+    masked, blocks = fmt_templates.mask(_SOURCE)
+    with pytest.raises(fmt_templates.MaskError):
+        fmt_templates.unmask(masked.replace("/* jinja:1 */", ""), blocks)
+    with pytest.raises(fmt_templates.MaskError):
+        fmt_templates.unmask(masked + "/* jinja:0 */\n", blocks)
+
+
+def test_every_jinja_component_masks_and_is_ignored_by_plain_oxfmt():
+    files = fmt_templates.jinja_components()
+    assert files, "no component holds Jinja: is the scan looking in the wrong place?"
+    ignored = fmt_templates.read_config()["ignorePatterns"]
+    for f in files:
+        source = (fmt_templates.REPO / f).read_text(encoding="utf-8")
+        masked, blocks = fmt_templates.mask(source)
+        assert blocks, f
+        assert fmt_templates.unmask(masked, blocks) == source, f
+        assert f in ignored, f"{f} holds Jinja but plain oxfmt would parse it"
diff --git a/uv.lock b/uv.lock
index d155eb6c..0fab3a99 100644
--- a/uv.lock
+++ b/uv.lock
@@ -185,6 +185,7 @@ docs = [
     { name = "mkdocs-material" },
 ]
 js = [
+    { name = "oxfmt" },
     { name = "oxlint" },
 ]
 
@@ -227,7 +228,10 @@ docs = [
     { name = "mkdocs-gen-files", specifier = ">=0.5.0" },
     { name = "mkdocs-material", specifier = ">=9.5.0" },
 ]
-js = [{ name = "oxlint", specifier = "==1.84.0" }]
+js = [
+    { name = "oxfmt", specifier = "==0.69.0" },
+    { name = "oxlint", specifier = "==1.84.0" },
+]
 
 [[package]]
 name = "claude-code-log-clmail-test"
@@ -793,6 +797,20 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/d2/1d/1b658dbd2b9fa9c4c9f32accbfc0205d532c8c6194dc0f2a4c0428e7128a/nodeenv-1.9.1-py2.py3-none-any.whl", hash = "sha256:ba11c9782d29c27c70ffbdda2d7415098754709be8a7056d79a737cd901155c9", size = 22314, upload-time = "2024-06-04T18:44:08.352Z" },
 ]
 
+[[package]]
+name = "oxfmt"
+version = "0.69.0"
+source = { registry = "https://pypi.org/simple" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/9b/14/11dbcb57f22391905fd18007d8113fa9be36540f6be3a53325c28eb670ea/oxfmt-0.69.0-py3-none-macosx_10_12_x86_64.whl", hash = "sha256:aa4e67c29a42908e9971fb2b0a6295dd8567d194f81738d7dfa95149591dba91", size = 2682208, upload-time = "2026-09-22T10:51:27.561Z" },
+    { url = "https://files.pythonhosted.org/packages/bc/a7/dafa3523f74ff1538aec64b96180bdf865254874812f17bf756f7bf76991/oxfmt-0.69.0-py3-none-macosx_11_0_arm64.whl", hash = "sha256:27b91eac1b4c3db94c0aca38bd405479bba104fc2132f3634e973f58124302bd", size = 2404020, upload-time = "2026-09-22T10:51:29.11Z" },
+    { url = "https://files.pythonhosted.org/packages/4c/77/817b20aba5172cdf4f597f6fa686aa89ebcc7f42a565410b6fe8d077c60d/oxfmt-0.69.0-py3-none-manylinux_2_18_aarch64.whl", hash = "sha256:338c3c27eee86acaf12c5d8846d795e32be29a8c94d7774d6af4f204f3755549", size = 2589393, upload-time = "2026-09-22T10:51:30.642Z" },
+    { url = "https://files.pythonhosted.org/packages/ca/09/f5f403fdd1da7d4103adf5176317f2c8e6dc6e583761acf55f829a616b7b/oxfmt-0.69.0-py3-none-manylinux_2_18_x86_64.whl", hash = "sha256:06f0f6a1acf7c7ea083f1ef2a576e66e90cadab9fc5b86f749f43e1aa6147264", size = 2789527, upload-time = "2026-09-22T10:51:31.982Z" },
+    { url = "https://files.pythonhosted.org/packages/bd/e8/52c2758989bc57ec1c250e7fb3a7f2fcfe0b12d78a760138525aa98626e6/oxfmt-0.69.0-py3-none-musllinux_1_2_aarch64.whl", hash = "sha256:0a013467b62240418c87253c3581c6ba0a6ce05d11f04208cba5261c7295e07e", size = 2625406, upload-time = "2026-09-22T10:51:33.772Z" },
+    { url = "https://files.pythonhosted.org/packages/0e/27/d5e2847c17ebb81fb253d89d124e5a77ad87ff0b6f7c61ea9a85a5663967/oxfmt-0.69.0-py3-none-musllinux_1_2_x86_64.whl", hash = "sha256:07a6f8f3a6807e75e0648a86f2be269fa7a2ad4db4a0ba2c7bc9f5a5a55d656c", size = 2847104, upload-time = "2026-09-22T10:51:35.291Z" },
+    { url = "https://files.pythonhosted.org/packages/96/d5/5259971861dbaae430dd094a96f442cf1a4c911826587fa1a14eeb192d63/oxfmt-0.69.0-py3-none-win_amd64.whl", hash = "sha256:c05119fb81e25d37a81f0a784f39c19fd3e4a857555495189c6f8c6427e751c5", size = 2568650, upload-time = "2026-09-22T10:51:36.707Z" },
+]
+
 [[package]]
 name = "oxlint"
 version = "1.84.0"