Treat image labels as Markdown alternative text

internal/validation/markdown.go

@@ -82,6 +82,8 @@ func parseMarkdown(source []byte) *markdownDocument {
 				Reference:   node.Reference != nil,
 				Location:    markdownLocationAt(lineStarts, node.Pos()),
 			})
+			// Image labels contribute alternative text, not navigable destinations.
+			return ast.WalkSkipChildren, nil
 		case *ast.AutoLink:
 			document.links = append(document.links, markdownLink{
 				Destination: node.Destination.Value(source),

internal/validation/markdown_check_test.go

@@ -46,6 +46,39 @@ func TestCheckMarkdownLinksResolvesCommittedDestinationsAndFragments(t *testing.
 	}
 }
 
+func TestCheckMarkdownLinksTreatsImageLabelsAsAlternativeText(t *testing.T) {
+	_, tree := committedMarkdownTree(t, map[string][]byte{
+		"guide.md": []byte("" +
+			"![![inner](missing.png)](image.png)\n" +
+			"![[label](missing.md)](image.png)\n" +
+			"![[anchor](target.md#absent)](image.png)\n" +
+			"![[outside](../outside.md)](image.png)\n" +
+			"[![logo](image.png)](target.md#present)\n" +
+			"[![missing](absent.png)](absent.md)\n"),
+		"image.png": []byte("image"),
+		"target.md": []byte("# Present\n"),
+	})
+	for _, enforceBoundary := range []bool{false, true} {
+		t.Run(fmt.Sprintf("boundary=%t", enforceBoundary), func(t *testing.T) {
+			err := checkMarkdownLinks(context.Background(), tree, enforceBoundary, nil)
+			var failure *Failure
+			if !errors.As(err, &failure) || failure.Check != MarkdownLinks || len(failure.Findings) != 2 {
+				t.Fatalf("error = %v, want only the missing linked-image source and link target", err)
+			}
+			want := map[string]bool{"absent.png": true, "absent.md": true}
+			for _, finding := range failure.Findings {
+				if !want[finding.Target] {
+					t.Errorf("unexpected destination finding: %#v", finding)
+				}
+				delete(want, finding.Target)
+			}
+			if len(want) != 0 {
+				t.Errorf("missing findings for real destinations: %v", want)
+			}
+		})
+	}
+}
+
 func TestCheckMarkdownLinksReportsMissingPathsAnchorsAndCycles(t *testing.T) {
 	_, tree := committedMarkdownTree(t, map[string][]byte{
 		"guide.md": []byte("" +