Add heading offset option to HTML renderer (#421)

Setting HTMLRendererParameters.HeadingLevelOffset to L changes
default HTML heading levels from hX to h(X+L).

block_test.go

@@ -371,6 +371,92 @@ func TestPrefixAutoHeaderIdExtensionWithPrefixAndSuffix(t *testing.T) {
 	})
 }
 
+func TestPrefixHeaderLevelOffset(t *testing.T) {
+	var offsetTests = []struct {
+		offset int
+		tests  []string
+	}{{
+		offset: 0,
+		tests: []string{
+			"# Header 1\n",
+			"<h1>Header 1</h1>\n",
+
+			"## Header 2\n",
+			"<h2>Header 2</h2>\n",
+
+			"### Header 3\n",
+			"<h3>Header 3</h3>\n",
+
+			"#### Header 4\n",
+			"<h4>Header 4</h4>\n",
+
+			"##### Header 5\n",
+			"<h5>Header 5</h5>\n",
+
+			"###### Header 6\n",
+			"<h6>Header 6</h6>\n",
+
+			"####### Header 7\n",
+			"<h6># Header 7</h6>\n",
+		},
+	}, {
+		offset: 1,
+		tests: []string{
+			"# Header 1\n",
+			"<h2>Header 1</h2>\n",
+
+			"## Header 2\n",
+			"<h3>Header 2</h3>\n",
+
+			"### Header 3\n",
+			"<h4>Header 3</h4>\n",
+
+			"#### Header 4\n",
+			"<h5>Header 4</h5>\n",
+
+			"##### Header 5\n",
+			"<h6>Header 5</h6>\n",
+
+			"###### Header 6\n",
+			"<h6>Header 6</h6>\n",
+
+			"####### Header 7\n",
+			"<h6># Header 7</h6>\n",
+		},
+	}, {
+		offset: -1,
+		tests: []string{
+			"# Header 1\n",
+			"<h1>Header 1</h1>\n",
+
+			"## Header 2\n",
+			"<h1>Header 2</h1>\n",
+
+			"### Header 3\n",
+			"<h2>Header 3</h2>\n",
+
+			"#### Header 4\n",
+			"<h3>Header 4</h3>\n",
+
+			"##### Header 5\n",
+			"<h4>Header 5</h4>\n",
+
+			"###### Header 6\n",
+			"<h5>Header 6</h5>\n",
+
+			"####### Header 7\n",
+			"<h5># Header 7</h5>\n",
+		},
+	}}
+	for _, offsetTest := range offsetTests {
+		offset := offsetTest.offset
+		tests := offsetTest.tests
+		doTestsParam(t, tests, TestParams{
+			HTMLRendererParameters: HTMLRendererParameters{HeadingLevelOffset: offset},
+		})
+	}
+}
+
 func TestPrefixMultipleHeaderExtensions(t *testing.T) {
 	var tests = []string{
 		"# Header\n\n# Header {#header}\n\n# Header 1",

html.go

@@ -87,6 +87,10 @@ type HTMLRendererParameters struct {
 	HeadingIDPrefix string
 	// If set, add this text to the back of each Heading ID, to ensure uniqueness.
 	HeadingIDSuffix string
+	// Increase heading levels: if the offset is 1, <h1> becomes <h2> etc.
+	// Negative offset is also valid.
+	// Resulting levels are clipped between 1 and 6.
+	HeadingLevelOffset int
 
 	Title string // Document title (used if CompletePage is set)
 	CSS   string // Optional CSS file URL (used if CompletePage is set)
@@ -460,9 +464,10 @@ var (
 )
 
 func headingTagsFromLevel(level int) ([]byte, []byte) {
-	switch level {
-	case 1:
+	if level <= 1 {
 		return h1Tag, h1CloseTag
+	}
+	switch level {
 	case 2:
 		return h2Tag, h2CloseTag
 	case 3:
@@ -471,9 +476,8 @@ func headingTagsFromLevel(level int) ([]byte, []byte) {
 		return h4Tag, h4CloseTag
 	case 5:
 		return h5Tag, h5CloseTag
-	default:
-		return h6Tag, h6CloseTag
 	}
+	return h6Tag, h6CloseTag
 }
 
 func (r *HTMLRenderer) outHRTag(w io.Writer) {
@@ -651,7 +655,8 @@ func (r *HTMLRenderer) RenderNode(w io.Writer, node *Node, entering bool) WalkSt
 		r.out(w, node.Literal)
 		r.cr(w)
 	case Heading:
-		openTag, closeTag := headingTagsFromLevel(node.Level)
+		headingLevel := r.HTMLRendererParameters.HeadingLevelOffset + node.Level
+		openTag, closeTag := headingTagsFromLevel(headingLevel)
 		if entering {
 			if node.IsTitleblock {
 				attrs = append(attrs, `class="title"`)