diff --git a/README.md b/README.md
index 05da79f..2d0e826 100644
--- a/README.md
+++ b/README.md
@@ -417,6 +417,7 @@ go build -o markdown-proxy ./cmd/markdown-proxy
 - **GitHub/GitLab branch detection**: When accessing a repository root URL, only `main` and `master` branches are tried for README.md auto-detection.
 - **No native PDF export**: Use the toolbar's Print link to export via the browser's print-to-PDF feature. Page breaks are automatically avoided inside tables, code blocks, math expressions, images, blockquotes, and list items; headings are kept together with the following content.
 - **Hidden files excluded**: Files and directories starting with `.` are not shown in directory listings.
+- **List marker spacing is normalized**: Under strict CommonMark/GFM, a list marker (e.g. `1.`) followed by 5 or more spaces turns the rest of the line into an indented code block instead of plain text — an easy mistake to make when padding markers for visual alignment. This proxy always normalizes the spacing after a list marker to a single space, so `1.     a` renders the same as `1. a`. This is an intentional deviation from GitHub's rendering. The normalization is applied per line, so deeply nested list items (indented past 3 columns) may not be covered.
 
 ## Contributing
 
diff --git a/internal/markdown/convert.go b/internal/markdown/convert.go
index 9b09c6e..d6eb0bd 100644
--- a/internal/markdown/convert.go
+++ b/internal/markdown/convert.go
@@ -53,6 +53,10 @@ func Convert(source []byte, plantumlServer string) ([]byte, error) {
 	// Normalize CRLF to LF so all preprocessors and the parser see consistent line endings.
 	source = bytes.ReplaceAll(source, []byte("\r\n"), []byte("\n"))
 
+	// Pre-process: normalize spacing after list markers to avoid accidental
+	// indented-code-block interpretation (see PreprocessListMarkers doc comment)
+	source = PreprocessListMarkers(source)
+
 	// Pre-process: expand single-line $$...$$ to multi-line for goldmark-mathjax
 	source = PreprocessMathBlocks(source)
 
diff --git a/internal/markdown/listmarker.go b/internal/markdown/listmarker.go
new file mode 100644
index 0000000..6c2d714
--- /dev/null
+++ b/internal/markdown/listmarker.go
@@ -0,0 +1,73 @@
+package markdown
+
+import (
+	"bytes"
+	"regexp"
+)
+
+// listMarkerRe matches a list item marker (bullet or ordered) at the start of
+// a line's content, followed by 2 or more spaces before the item's text.
+// Capture groups: (1) leading indent (0-3 spaces), (2) marker, (3) content.
+// The marker itself may only be preceded by 0-3 spaces, matching CommonMark's
+// own rule for what counts as a (possibly nested) list marker rather than
+// indented content.
+var listMarkerRe = regexp.MustCompile(`^( {0,3})([-*+]|\d{1,9}[.)]) {2,}(\S.*)$`)
+
+// PreprocessListMarkers normalizes the whitespace between a list marker and
+// its content to exactly one space.
+//
+// CommonMark/GFM caps the "content indent" a list item establishes at 4
+// columns: if a marker is followed by 5 or more spaces, only the first space
+// is treated as the separator and the rest is read as part of the item's
+// text, which then renders as an indented code block (4+ columns of leading
+// whitespace) instead of plain text. Authors who pad list markers for visual
+// alignment (e.g. "1.     a") trigger this by accident and get a silently
+// different rendering. Normalizing the spacing up front removes the
+// ambiguity: a list marker is always followed by exactly one space,
+// regardless of how the source was typed.
+//
+// This only rewrites the marker's own line, so intentional multi-line
+// indented content (nested code blocks, continuation paragraphs) on
+// following lines is unaffected. Fenced code blocks (``` or ~~~) are left
+// untouched, and only markers preceded by 0-3 spaces are rewritten, so
+// already-indented code content is never touched.
+//
+// Deviation from strict CommonMark/GFM: this is an intentional divergence
+// from GitHub's rendering, favoring predictability over spec compliance for
+// this specific, easily-mistaken construct.
+func PreprocessListMarkers(source []byte) []byte {
+	lines := bytes.Split(source, []byte("\n"))
+	var inFence bool
+	var fenceMarker []byte
+
+	for i, line := range lines {
+		bqPrefix, body := splitBlockquotePrefix(line)
+
+		if m := fenceStartRe.FindSubmatch(body); m != nil {
+			marker := m[1]
+			if inFence {
+				if marker[0] == fenceMarker[0] && len(marker) >= len(fenceMarker) {
+					inFence = false
+					fenceMarker = nil
+				}
+			} else {
+				inFence = true
+				fenceMarker = marker
+			}
+			continue
+		}
+		if inFence {
+			continue
+		}
+
+		if m := listMarkerRe.FindSubmatch(body); m != nil {
+			normalized := append([]byte{}, m[1]...) // leading indent
+			normalized = append(normalized, m[2]...) // marker
+			normalized = append(normalized, ' ')
+			normalized = append(normalized, m[3]...) // content
+			lines[i] = prefixed(bqPrefix, normalized)
+		}
+	}
+
+	return bytes.Join(lines, []byte("\n"))
+}
diff --git a/internal/markdown/listmarker_test.go b/internal/markdown/listmarker_test.go
new file mode 100644
index 0000000..9d8bcbb
--- /dev/null
+++ b/internal/markdown/listmarker_test.go
@@ -0,0 +1,103 @@
+package markdown
+
+import (
+	"testing"
+)
+
+func TestPreprocessListMarkers(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		expect string
+	}{
+		{
+			name:   "ordered marker with 5 spaces (the reported bug)",
+			input:  "1.     a",
+			expect: "1. a",
+		},
+		{
+			name:   "ordered marker with 2 spaces stays normalized",
+			input:  "1.  a",
+			expect: "1. a",
+		},
+		{
+			name:   "ordered marker with 1 space is unchanged",
+			input:  "1. a",
+			expect: "1. a",
+		},
+		{
+			name:   "bullet marker with many spaces",
+			input:  "-      a",
+			expect: "- a",
+		},
+		{
+			name:   "plus and asterisk bullets",
+			input:  "*      a\n+      b",
+			expect: "* a\n+ b",
+		},
+		{
+			name:   "two-digit ordered marker",
+			input:  "10.     a",
+			expect: "10. a",
+		},
+		{
+			name:   "paren-style ordered marker",
+			input:  "1)     a",
+			expect: "1) a",
+		},
+		{
+			name:   "shallow nested bullet under bullet",
+			input:  "- a\n  -     b",
+			expect: "- a\n  - b",
+		},
+		{
+			name:   "leading indent up to 3 spaces preserved",
+			input:  "   -     a",
+			expect: "   - a",
+		},
+		{
+			name:   "internal spacing within content untouched",
+			input:  "1.     a     b",
+			expect: "1. a     b",
+		},
+		{
+			name:   "marker-only empty item untouched",
+			input:  "-   \nfoo",
+			expect: "-   \nfoo",
+		},
+		{
+			name:   "content inside fenced code block untouched",
+			input:  "```\n1.     a\n```",
+			expect: "```\n1.     a\n```",
+		},
+		{
+			name:   "content inside tilde fenced code block untouched",
+			input:  "~~~\n1.     a\n~~~",
+			expect: "~~~\n1.     a\n~~~",
+		},
+		{
+			name:   "indented (4+ space) code block untouched",
+			input:  "    1.     a",
+			expect: "    1.     a",
+		},
+		{
+			name:   "list marker inside blockquote normalized",
+			input:  "> 1.     a",
+			expect: "> 1. a",
+		},
+		{
+			name:   "thematic break with wide spacing survives as a thematic break",
+			input:  "-   -   -",
+			expect: "- -   -",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := string(PreprocessListMarkers([]byte(tt.input)))
+			if got != tt.expect {
+				t.Errorf("PreprocessListMarkers(%q) = %q, want %q", tt.input, got, tt.expect)
+			}
+		})
+	}
+}