diff --git a/docs/dialects.md b/docs/dialects.md
index 36a1579..71c6264 100644
--- a/docs/dialects.md
+++ b/docs/dialects.md
@@ -114,11 +114,11 @@ flowchart TD
     IN["filter text"] --> LEX["Lexer: tokens<br/>(strings read as the dialect reads them)"]
     LEX --> OP["an operator, as written<br/>(e.g. !=, ILIKE, ~~, ISNULL)"]
     OP --> D1{"does the dialect have<br/>the operator?"}
-    D1 -->|"no: ILIKE, ~ in SQL:2023 and MySQL 8.4;<br/>SIMILAR TO, IS DISTINCT FROM in MySQL 8.4"| E1["operator &quot;ILIKE&quot; is not part of<br/>the SQL:2023 dialect"]
+    D1 -->|"no: ILIKE, ~ in SQL:2023 and MySQL 8.4;<br/>SIMILAR TO, IS DISTINCT FROM in MySQL 8.4"| E1["operator #quot;ILIKE#quot; is not part of<br/>the SQL:2023 dialect"]
     D1 -->|"yes"| D2{"does it have<br/>this spelling of it?"}
-    D2 -->|"no: != in SQL:2023; ~~ !~~ ISNULL NOTNULL<br/>in both; BETWEEN SYMMETRIC in MySQL 8.4"| E2["!= is not part of the SQL:2023<br/>dialect: write &lt;&gt;"]
+    D2 -->|"no: != in SQL:2023; ~~ !~~ ISNULL NOTNULL<br/>in both; BETWEEN SYMMETRIC in MySQL 8.4"| E2["!= is not part of the SQL:2023<br/>dialect: write #lt;#gt;"]
     D2 -->|"yes"| A{"allowed by<br/>WithAllowedOperators?"}
-    A -->|"no"| E3["operator &quot;LIKE&quot; is not allowed"]
+    A -->|"no"| E3["operator #quot;LIKE#quot; is not allowed"]
     A -->|"yes, or no allow-list"| OK["into the AST"]
     E1 --> J["errors.Join: every problem"]
     E2 --> J
diff --git a/docs/error-handling.md b/docs/error-handling.md
index b605142..ac27558 100644
--- a/docs/error-handling.md
+++ b/docs/error-handling.md
@@ -60,7 +60,7 @@ flowchart TD
     P --> CONS{"entire input<br/>consumed?"}
     CONS -->|"no"| ADD["add 'unexpected token' error"]
     CONS -->|"yes"| Q
-    ADD --> Q{"len(errors) &gt; 0?"}
+    ADD --> Q{"len(errors) #gt; 0?"}
     Q -->|"yes"| J["return errors.Join(…)<br/>of *QFVFilterError"]
     Q -->|"no"| OK["return AST, nil"]
 ```
diff --git a/docs/filtering.md b/docs/filtering.md
index b3f95da..447f2b6 100644
--- a/docs/filtering.md
+++ b/docs/filtering.md
@@ -273,7 +273,7 @@ parses into this tree (precedence makes `AND` the root, with the parenthesized
 graph TD
     AND["BinaryOperatorNode<br/>AND"]
     AND --> GRP["GroupNode<br/>( … )"]
-    AND --> GT["BinaryOperatorNode<br/>&gt;"]
+    AND --> GT["BinaryOperatorNode<br/>#gt;"]
 
     GRP --> OR["BinaryOperatorNode<br/>OR"]
     OR --> EQ1["BinaryOperatorNode<br/>="]
diff --git a/docs_mermaid_test.go b/docs_mermaid_test.go
new file mode 100644
index 0000000..27c21c3
--- /dev/null
+++ b/docs_mermaid_test.go
@@ -0,0 +1,50 @@
+package qfv
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"testing"
+)
+
+// GitHub decodes HTML entities before it hands a diagram to Mermaid, so a
+// label written with &quot; reaches Mermaid as a quote that ends the label:
+// docs/dialects.md's checking diagram rendered as a parse error on GitHub,
+// and as a diagram everywhere else. Mermaid's own entity codes (#quot;,
+// #lt;, #gt;, #amp;) are left alone by GitHub and decoded by Mermaid.
+func TestNoDiagramUsesAnHTMLEntity(t *testing.T) {
+	t.Parallel()
+
+	files, err := filepath.Glob("*.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	docs, err := filepath.Glob("docs/*.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	block := regexp.MustCompile("(?s)```mermaid\n(.*?)```")
+	entity := regexp.MustCompile(`&(quot|lt|gt|amp|#[0-9]+);`)
+	diagrams := 0
+
+	for _, f := range append(files, docs...) {
+		raw, err := os.ReadFile(f)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		for _, m := range block.FindAllStringSubmatch(string(raw), -1) {
+			diagrams++
+
+			if e := entity.FindString(m[1]); e != "" {
+				t.Errorf("%s: a diagram uses the HTML entity %s, which GitHub decodes before Mermaid parses it: write #%s", f, e, e[1:])
+			}
+		}
+	}
+
+	if diagrams < 10 {
+		t.Fatalf("read %d diagrams, want at least 10: the docs moved and this test reads nothing", diagrams)
+	}
+}