Add github action to tidy the spec

close #2558

.github/workflows/tidy.yml

@@ -0,0 +1,24 @@
+name: tidy-up spec
+
+on: workflow_dispatch
+
+jobs:
+  tidy:
+      runs-on: ubuntu-latest
+      steps:
+        - uses: actions/checkout@v2
+        - name: Download HTML5 Tidy 5.4
+          run: wget https://github.com/htacg/tidy-html5/releases/download/5.4.0/tidy-5.4.0-64bit.deb
+        - name: Install tidy
+          sudo dpkg -i tidy-5.4.0-64bit.deb
+        - name: Run it
+          tidy --char-encoding utf8 --indent yes --indent-spaces 2 --wrap 80 --tidy-mark no -m webrtc.html
+      - uses: peter-evans/create-pull-request@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.W3C_GITHUB_TOKEN }}
+        with:
+          title: Fix indenting
+          commit-message: "With indenting with tidy"
+          draft: true
+          labels: Editorial
+          branch: tidy-${{GITHUB_RUN_NUMBER}}