FIX HTML PROOFER. (#2)

Finally

Author: lwasser

.github/workflows/build-book.yml

@@ -1,7 +1,14 @@
 name: build-test-deploy-book
 
-# Run on pushes and pull requests
-on: [push, pull_request]
+# Only build PRs, the main branch, and releases. Pushes to branches will only
+# be built when a PR is opened. This avoids duplicated buids in PRs comming
+# from branches in the origin repository (1 for PR and 1 for push).
+# This came from Leo's work with fatiando
+on:
+  pull_request:
+  push:
+    branches:
+      - main
 
 # This job installs dependencies, build the book, and pushes it to `gh-pages`
 jobs:
@@ -53,3 +60,13 @@ jobs:
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         publish_dir: ./_build/html
+
+    # Test for bad links and ensure alt tags for usability
+    - name: Check HTML using htmlproofer
+      uses: chabad360/htmlproofer@master
+      with:
+        directory: '_build/html'
+        arguments: |
+          --ignore-files "/.+\/_static\/.+/"
+
+