Add htmltest github action

.github/workflows/main.yml

@@ -142,6 +142,28 @@ jobs:
           name: spec-artifact
           path: spec.tar.gz
 
+  htmlcheck:
+    name: "🔎 Validate generated HTML"
+    runs-on: ubuntu-latest
+    needs: [build-spec]
+    steps:
+      - name: "📥 Source checkout"
+        uses: actions/checkout@v2
+
+      - name: "📥 Fetch built spec"
+        uses: actions/download-artifact@v2
+        with:
+          name: spec-artifact
+
+      - name: "📝 Unpack the spec"
+        run: |
+          tar -xvzf spec.tar.gz
+
+      - name: "Run htmltest"
+        uses: wjdp/htmltest-action@master
+        with:
+          config: .htmltest.yaml
+
   build-historical-spec:
     name: "📖 Build the historical backup spec"
     runs-on: ubuntu-latest