fix: don't de-indent HTML inside fenced code blocks

CHANGELOG.md

@@ -16,6 +16,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **GitHub Metadata Cache** (#43): Reuse successful GitHub repository metadata requests for ten minutes and coalesce concurrent requests
 
+### Fixed
+
+- **HTML De-indenting in Code Fences**: `deIndentHTMLBlocks` no longer strips indentation from HTML-looking lines inside fenced code blocks, which mangled indented HTML code samples
+
 ## [0.3.1] - 2026-02-27
 
 ### Fixed

renderers/markdown.go

@@ -51,12 +51,67 @@ func createGoldmarkConverter() goldmark.Markdown {
 // (optionally preceded by up to 3 spaces) and ends at a blank line.
 var htmlBlockStartRE = regexp.MustCompile(`(?i)^\s{0,3}</?(?:address|article|aside|blockquote|details|dialog|dd|div|dl|dt|fieldset|figcaption|figure|footer|form|h[1-6]|header|hgroup|hr|li|main|nav|ol|p|pre|section|summary|table|ul)\b`)
 
+// codeFenceDelimiter returns the marker character, length, and trailing
+// content of a fenced code block delimiter. Fenced code blocks are left
+// untouched, so that HTML-looking code samples keep their indentation.
+func codeFenceDelimiter(line []byte) (byte, int, []byte, bool) {
+	indent := 0
+	for indent < len(line) && indent < 4 && line[indent] == ' ' {
+		indent++
+	}
+	if indent == len(line) || indent == 4 {
+		return 0, 0, nil, false
+	}
+
+	marker := line[indent]
+	if marker != '`' && marker != '~' {
+		return 0, 0, nil, false
+	}
+
+	end := indent
+	for end < len(line) && line[end] == marker {
+		end++
+	}
+	length := end - indent
+	if length < 3 {
+		return 0, 0, nil, false
+	}
+	return marker, length, line[end:], true
+}
+
+func isCodeFenceClosingSuffix(suffix []byte) bool {
+	if len(suffix) > 0 && suffix[len(suffix)-1] == '\r' {
+		suffix = suffix[:len(suffix)-1]
+	}
+	return len(bytes.Trim(suffix, " \t")) == 0
+}
+
 func deIndentHTMLBlocks(md []byte) []byte {
 	lines := bytes.Split(md, []byte("\n"))
 	result := make([][]byte, 0, len(lines))
 	inHTMLBlock := false
+	inCodeFence := false
+	var fenceMarker byte
+	fenceLength := 0
 
 	for _, line := range lines {
+		marker, length, suffix, isFence := codeFenceDelimiter(line)
+		if inCodeFence {
+			if isFence && marker == fenceMarker && length >= fenceLength && isCodeFenceClosingSuffix(suffix) {
+				inCodeFence = false
+				fenceMarker = 0
+				fenceLength = 0
+			}
+			result = append(result, line)
+			continue
+		}
+		if isFence {
+			inCodeFence = true
+			fenceMarker = marker
+			fenceLength = length
+			result = append(result, line)
+			continue
+		}
 		if !inHTMLBlock {
 			if htmlBlockStartRE.Match(line) {
 				inHTMLBlock = true

renderers/markdown_test.go

@@ -93,6 +93,56 @@ func TestDeIndentHTMLBlocks(t *testing.T) {
 				require.Contains(t, result, "    outside")
 			},
 		},
+		{
+			name:  "HTML inside backtick code fence untouched",
+			input: "```\n<ul>\n    <li>item</li>\n</ul>\n```\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "    <li>item</li>")
+			},
+		},
+		{
+			name:  "HTML inside tilde code fence untouched",
+			input: "~~~\n<div>\n    indented\n</div>\n~~~\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "    indented")
+			},
+		},
+		{
+			name:  "longer outer fence containing shorter fence lines",
+			input: "````html\n```\n<ul>\n    <li>item</li>\n</ul>\n```\n````\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "    <li>item</li>")
+			},
+		},
+		{
+			name:  "backtick fence containing tilde fence lines",
+			input: "```\n~~~\n<ul>\n    <li>item</li>\n</ul>\n~~~\n```\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "    <li>item</li>")
+			},
+		},
+		{
+			name:  "fence-like content with trailing text does not close",
+			input: "```\n```not-a-close\n<ul>\n    <li>item</li>\n</ul>\n```\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "    <li>item</li>")
+			},
+		},
+		{
+			name:  "closing fence allows whitespace",
+			input: "```\r\ncode\r\n``` \t\r\n<ul>\r\n    <li>item</li>\r\n</ul>\r\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "\r\n<li>item</li>")
+			},
+		},
+		{
+			name:  "HTML block after code fence still de-indented",
+			input: "```\n<p>    code</p>\n```\n\n<ul>\n    <li>item</li>\n</ul>\n",
+			check: func(t *testing.T, result string) {
+				require.Contains(t, result, "\n<li>item</li>")
+				require.Contains(t, result, "<p>    code</p>")
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -102,6 +152,15 @@ func TestDeIndentHTMLBlocks(t *testing.T) {
 	}
 }
 
+func TestRenderMarkdownCodeFenceIndentation(t *testing.T) {
+	// HTML code samples inside fenced code blocks must keep their
+	// indentation; deIndentHTMLBlocks previously stripped it.
+	input := "```html\n<ul>\n    <li>item</li>\n</ul>\n```\n"
+	result := mustMarkdownString(input)
+	require.Contains(t, result, "    &lt;li&gt;item&lt;/li&gt;",
+		"code sample should keep its indentation")
+}
+
 func TestRenderMarkdownVoidElements(t *testing.T) {
 	// Issue #66: <br> tags inside markdown="1" blocks should not cause EOF errors.
 	// Void elements like <br>, <hr>, <img> don't have end tags, so the depth