diff --git a/.travis.yml b/.travis.yml
index 758927e..fef41c7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,11 @@ install:
   - pip install --upgrade pip
   - pip install ruamel.yaml
 
+addons:
+  apt:
+    packages:
+    - libcurl4-openssl-dev # required to avoid SSL error (for htmlproofer)
+
 script:
   - bash _scripts/travis/cibuild.sh
 
@@ -19,7 +24,8 @@ git:
 
 cache:
   directories:
-    - vendor/bundle
+    - $TRAVIS_BUILD_DIR/vendor/bundle
+    - $TRAVIS_BUILD_DIR/tmp/.htmlproofer
 
 notifications:
   email:
@@ -29,4 +35,5 @@ notifications:
 
 env:
   global:
+    - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
     - secure: "QNAqDoDQ+mF/3DFxKbc7fnYa5+FysJTX9Vs4XneH6iH63MS2lNtHfC7uT/M0/sRJaygK9eAdb+RaIeADE7CAMzqP3qdWx7l2N6qWab/kyK9KujUBsvoTGX6lDQTwC2Z+zlr5uUScHfLOhqHh6dbm6glziy2NB4dn9pVBXQrmBn09Rf9LImRT2u2h8gzmdl19hZ9TsW1/ROOb/1Soc3JElGRPrIxKI7ufHLQZt0UzZkYXGejQJwtstEEkRSmhfkJjlASeaeuyOFBvkWXjL1nKPsElwEKsAR+4II98JKA83YVqa9TJT2YSnXeLEFIhYDVSG6RLAMEU4rrITzRYC/hWKBKje4G0PDEM/cpnytk9Xhq+7oUFRmXCq3ZQgTQgzWhzl1kExC8b96hiWmQ+lVCw+Q2BG6+NloEEuOHOM839CTXfiitjRJ8KYUdLgW6IaruaingySBsbh1Vl+Dhu73QZ5H40KLdIH6woLXWAgtxkjpaL6DTeWDO6xMiwDreBa/2CJRT5R1tiG45CurZ+AkQ+yAAfSOEwGtE29oZl0sWeWMwVTaAClc0IJaG1+h8jxIVuQqHvK3fzd5RdlDtShrfCCVvQD0n29wXrdeNEg90F92kvdtmw8XgKc2P/Cl5uNzvVeTbFF3ZIHYpCVV4LW4Hn8clgYu+WUa/w4pVrBSdQOUw="
\ No newline at end of file
diff --git a/Gemfile b/Gemfile
index 73267dd..7103984 100644
--- a/Gemfile
+++ b/Gemfile
@@ -9,5 +9,9 @@ group :jekyll_plugins do
   gem 'jekyll-seo-tag'
 end
 
+group :test do
+  gem 'html-proofer'
+end
+
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
diff --git a/_scripts/travis/cibuild.sh b/_scripts/travis/cibuild.sh
index 1d61786..1edda8b 100644
--- a/_scripts/travis/cibuild.sh
+++ b/_scripts/travis/cibuild.sh
@@ -17,7 +17,10 @@ if [[ $TRAVIS_PULL_REQUEST = "false" ]]; then # triggered by author
 
 else # triggered by Pull Request
 
+  SAFE_DOMAIN=cdn.jsdelivr.net
+
   bundle install --path vendor/bundle --quiet
+
   python _scripts/py/init_all.py
 
   build_cmd="JEKYLL_ENV=production bundle exec jekyll build"
@@ -25,4 +28,11 @@ else # triggered by Pull Request
   echo "\$ $build_cmd"
   eval $build_cmd
 
+  bundle exec htmlproofer --disable-external \
+            --check-html \
+            --empty_alt_ignore \
+            --allow_hash_href \
+            --url_ignore $SAFE_DOMAIN \
+            _site/
+
 fi