Enable HTML validation in CI

Author: eXpl0it3r

.github/workflows/build.yml

@@ -1,4 +1,4 @@
-name: Build
+name: Build, Validate, Package & Publish
 
 on: [push, pull_request, workflow_dispatch]
 
@@ -51,6 +51,13 @@ jobs:
     - name: Build
       run: dotnet build Source/HtmlRenderer.sln --configuration Release --no-restore
 
+    - name: Validate HTML
+      uses: anishathalye/proof-html@v2
+      with:
+        directory: ./Source
+        check_html: true
+        check_css: true
+
     - name: Create HtmlRenderer.Core NuGet package
       run: dotnet pack Source/HtmlRenderer/HtmlRenderer.csproj --configuration Release --include-symbols -p:SymbolPackageFormat=snupkg --no-build --verbosity normal --output ${{ env.NuGetDirectory }}