Unit test code to assert new header-footer are correctly registred

src/Html2OpenXml/HtmlConverter.cs

@@ -138,6 +138,12 @@ public async Task ParseHeader(string html, CancellationToken cancellationToken =
         if (partId != null)
         {
             var sectionProps = mainPart.Document.Body!.Elements<SectionProperties>();
+            if (!sectionProps.Any())
+            {
+                sectionProps = [new SectionProperties()];
+                mainPart.Document.Body!.AddChild(sectionProps.First());
+            }
+
             foreach (var sectPr in sectionProps)
             {
                 sectPr.RemoveAllChildren<HeaderReference>();
@@ -178,6 +184,12 @@ public async Task ParseFooter(string html, CancellationToken cancellationToken =
         if (partId != null)
         {
             var sectionProps = mainPart.Document.Body!.Elements<SectionProperties>();
+            if (!sectionProps.Any())
+            {
+                sectionProps = [new SectionProperties()];
+                mainPart.Document.Body!.AddChild(sectionProps.First());
+            }
+
             foreach (var sectPr in sectionProps)
             {
                 sectPr.RemoveAllChildren<FooterReference>();

test/HtmlToOpenXml.Tests/HeaderFooterTests.cs

@@ -0,0 +1,42 @@
+using NUnit.Framework;
+using DocumentFormat.OpenXml.Wordprocessing;
+
+namespace HtmlToOpenXml.Tests
+{
+    /// <summary>
+    /// Tests on <c>ParseHeader</c> and <c>ParseFooter</c> methods.
+    /// </summary>
+    [TestFixture]
+    public class HeaderFooterTests : HtmlConverterTestBase
+    {
+        [Test]
+        public async Task Header_ReturnsHeaderPartLinkedToBody()
+        {
+            await converter.ParseHeader("<p>Header content</p>");
+
+            var headerPart = mainPart.HeaderParts?.FirstOrDefault();
+            Assert.That(headerPart, Is.Not.Null);
+            Assert.That(headerPart.Header, Is.Not.Null);
+
+            var sectionProperties = mainPart.Document.Body!.Elements<SectionProperties>();
+            Assert.That(sectionProperties, Is.Not.Empty);
+            Assert.That(sectionProperties.Any(s => s.HasChild<HeaderReference>()), Is.True);
+            AssertThatOpenXmlDocumentIsValid();
+        }
+
+        [Test]
+        public async Task Footer_ReturnsFooterPartLinkedToBody()
+        {
+            await converter.ParseFooter("<p>Footer content</p>");
+
+            var footerPart = mainPart.FooterParts?.FirstOrDefault();
+            Assert.That(footerPart, Is.Not.Null);
+            Assert.That(footerPart.Footer, Is.Not.Null);
+
+            var sectionProperties = mainPart.Document.Body!.Elements<SectionProperties>();
+            Assert.That(sectionProperties, Is.Not.Empty);
+            Assert.That(sectionProperties.Any(s => s.HasChild<FooterReference>()), Is.True);
+            AssertThatOpenXmlDocumentIsValid();
+        }
+    }
+}

test/HtmlToOpenXml.Tests/HtmlConverterTestBase.cs

@@ -1,5 +1,6 @@
 using DocumentFormat.OpenXml;
 using DocumentFormat.OpenXml.Packaging;
+using DocumentFormat.OpenXml.Validation;
 using DocumentFormat.OpenXml.Wordprocessing;
 using NUnit.Framework;
 
@@ -36,5 +37,21 @@ public void Close ()
             package?.Dispose();
             generatedDocument?.Dispose();
         }
+
+        protected void AssertThatOpenXmlDocumentIsValid()
+        {
+            var validator = new OpenXmlValidator(FileFormatVersions.Office2021);
+            var errors = validator.Validate(package);
+
+            if (!errors.GetEnumerator().MoveNext())
+                return;
+
+            foreach (ValidationErrorInfo error in errors)
+            {
+                TestContext.Error.Write("{0}\n\t{1}\n", error.Path?.XPath, error.Description);
+            }
+
+            Assert.Fail("The document doesn't look 100% compatible with Office 2021");
+        }
     }
 }

test/HtmlToOpenXml.Tests/ImgTests.cs

@@ -188,6 +188,7 @@ public async Task ParseIntoDocumentPart_ReturnsImageParentedToPart (Type openXml
             var p = host.ChildElements.FirstOrDefault(c => c is Paragraph);
             Assert.That(p, Is.Not.Null);
             AssertIsImg(container, p);
+            AssertThatOpenXmlDocumentIsValid();
         }
 
         private static Drawing AssertIsImg (OpenXmlPartContainer container, OpenXmlElement paragraph)

test/HtmlToOpenXml.Tests/LinkTests.cs

@@ -179,6 +179,7 @@ public async Task ParseIntoDocumentPart_ReturnsHyperlinkParentedToPart (Type ope
             }
 
             AssertHyperlink(container, host.ChildElements);
+            AssertThatOpenXmlDocumentIsValid();
         }
 
         private static void AssertHyperlink(OpenXmlPartContainer container, IEnumerable<OpenXmlElement> elements)

test/HtmlToOpenXml.Tests/NumberingTests.cs

@@ -291,6 +291,7 @@ public async Task ContinueNumbering_ReturnsSecondList_ContinueOrder()
                 e.ParagraphProperties?.NumberingProperties?.NumberingId?.Val?.Value),
                 Has.All.EqualTo(instances.First().NumberID!.Value),
                 "All paragraphs are linked to the same list instance");
+            AssertThatOpenXmlDocumentIsValid();
         }
 
         [Test(Description = "Stop indenting from existing numbering (issue #57)")]
@@ -321,6 +322,7 @@ public async Task DisableContinueNumbering_ReturnsSecondList_RestartingOrder()
                 e.ParagraphProperties?.NumberingProperties?.NumberingId?.Val?.Value),
                 Is.Unique,
                 "All paragraphs use different list instances");
+            AssertThatOpenXmlDocumentIsValid();
         }
 
         /// <summary>