diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 4d2618998..7e1836f20 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -17,3 +17,17 @@ jobs:
 
     - name: Lint
       run: .ci/script.sh
+
+    # NPM tests
+
+    - name: Uses Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: 'latest'
+        # No cache, there are complaints about lockfiles that we don't want otherwise. See <https://github.com/actions/setup-node/issues/928>.
+        # cache: 'npm'
+
+    - run: npm init --yes && npm install stylelint stylelint-config-standard
+
+    - name: Check CSS syntax (only epub.css, html5.css and fb2.css)
+      run: npx stylelint cr3gui/data/epub.css cr3gui/data/html5.css cr3gui/data/fb2.css
diff --git a/.stylelintrc.json b/.stylelintrc.json
new file mode 100644
index 000000000..fa15d6b20
--- /dev/null
+++ b/.stylelintrc.json
@@ -0,0 +1,66 @@
+{
+  "extends": "stylelint-config-standard",
+  "rules": {
+    "comment-empty-line-before": null,
+    "declaration-block-no-redundant-longhand-properties": null,
+    "declaration-block-single-line-max-declarations": null,
+    "length-zero-no-unit": [true, { "severity": "warning" }],
+    "no-descending-specificity": [true, { "severity": "warning" }],
+    "no-duplicate-selectors": [true, { "severity": "warning" }],
+    "property-no-unknown": [true, {
+      "ignoreProperties": [
+        "binding",
+
+        "hyphenate"
+      ]
+    }],
+    "rule-empty-line-before": null,
+    "selector-attribute-quotes": null,
+    "selector-class-pattern": "(^([a-z][a-z0-9]*)(-[a-z0-9]+)*$|^fb2_info$)",
+    "selector-type-case": ["lower", {
+      "ignoreTypes": ["DocFragment", "rubyBox"]
+    }],
+    "selector-type-no-unknown": [true, {
+      "ignoreTypes": [
+        "noframes",
+        "rbc",
+
+        "DocFragment",
+        "rubyBox",
+
+        "empty-line",
+
+        "author",
+        "date",
+        "description",
+        "genre",
+
+        "epigraph",
+        "v",
+        "stanza",
+        "poem",
+        "text-author",
+        "epigraph",
+
+        "book-title",
+        "subtitle",
+        "title-info",
+        "keywords",
+        "lang",
+        "src-lang",
+        "translator",
+        "document-info",
+        "publish-info",
+        "custom-info",
+        "coverpage",
+        "underline",
+        "spacing",
+        "emphasis",
+        "underline",
+        "strikethrough",
+        "spacing"
+      ]
+    }],
+    "shorthand-property-no-redundant-values": null
+  }
+}
diff --git a/cr3gui/data/fb2.css b/cr3gui/data/fb2.css
index 25291ceea..06cb19dd1 100644
--- a/cr3gui/data/fb2.css
+++ b/cr3gui/data/fb2.css
@@ -43,10 +43,8 @@ title, h1, h2, h3, h4, h5, h6, subtitle {
 h1, h2, h3, h4, h5, h6 {
 	display: block;
 	margin-top: 0.5em;
-	margin-bottom: 0.3em;
-	padding: 10px ;
-	margin-top: 0.5em;
 	margin-bottom: 0.5em;
+	padding: 10px ;
 }
 title, h1, h2 {
 	page-break-before: always;