From 799317b788ecac9368bb8e53bd29f10ea6e16c1b Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Sat, 4 Apr 2026 01:31:38 +0000
Subject: [PATCH] feat(codegen): reject reserved custom element tags

Co-Authored-By: Virgil <virgil@lethean.io>
---
 cmd/wasm/components.go    | 13 +++++++++++++
 cmd/wasm/register_test.go |  1 +
 codegen/codegen.go        | 25 ++++++++++++++++++++++++-
 codegen/codegen_test.go   | 27 +++++++++++++++++++++++++++
 4 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/cmd/wasm/components.go b/cmd/wasm/components.go
index f9f5673..c447ab2 100644
--- a/cmd/wasm/components.go
+++ b/cmd/wasm/components.go
@@ -7,6 +7,16 @@ import (
 )
 
 var canonicalSlotOrder = []string{"H", "L", "C", "R", "F"}
+var reservedCustomElementTags = map[string]struct{}{
+	"annotation-xml":   {},
+	"color-profile":    {},
+	"font-face":        {},
+	"font-face-src":    {},
+	"font-face-uri":    {},
+	"font-face-format": {},
+	"font-face-name":   {},
+	"missing-glyph":    {},
+}
 
 // cmd/wasm/components.go: isValidCustomElementTag reports whether tag is a safe
 // custom element name.
@@ -19,6 +29,9 @@ func isValidCustomElementTag(tag string) bool {
 	if tag[0] < 'a' || tag[0] > 'z' {
 		return false
 	}
+	if _, reserved := reservedCustomElementTags[tag]; reserved {
+		return false
+	}
 
 	for i := range len(tag) {
 		ch := tag[i]
diff --git a/cmd/wasm/register_test.go b/cmd/wasm/register_test.go
index 313aa54..8cdcccb 100644
--- a/cmd/wasm/register_test.go
+++ b/cmd/wasm/register_test.go
@@ -50,6 +50,7 @@ func TestIsValidCustomElementTag(t *testing.T) {
 		{tag: "NavBar", want: false},
 		{tag: "nav", want: false},
 		{tag: "nav_bar", want: false},
+		{tag: "annotation-xml", want: false},
 	}
 
 	for _, tt := range tests {
diff --git a/codegen/codegen.go b/codegen/codegen.go
index 229276b..85e8a6e 100644
--- a/codegen/codegen.go
+++ b/codegen/codegen.go
@@ -11,6 +11,22 @@ import (
 
 // isValidCustomElementTag reports whether tag is a safe custom element name.
 // The generator rejects values that would fail at customElements.define() time.
+var reservedCustomElementTags = map[string]struct{}{
+	"annotation-xml":   {},
+	"color-profile":    {},
+	"font-face":        {},
+	"font-face-src":    {},
+	"font-face-uri":    {},
+	"font-face-format": {},
+	"font-face-name":   {},
+	"missing-glyph":    {},
+}
+
+func isReservedCustomElementTag(tag string) bool {
+	_, reserved := reservedCustomElementTags[tag]
+	return reserved
+}
+
 func isValidCustomElementTag(tag string) bool {
 	if tag == "" || !strings.Contains(tag, "-") {
 		return false
@@ -18,6 +34,9 @@ func isValidCustomElementTag(tag string) bool {
 	if tag[0] < 'a' || tag[0] > 'z' {
 		return false
 	}
+	if isReservedCustomElementTag(tag) {
+		return false
+	}
 
 	for i := range len(tag) {
 		ch := tag[i]
@@ -60,7 +79,11 @@ var wcTemplate = template.Must(template.New("wc").Parse(`class {{.ClassName}} ex
 // Example: cls, err := GenerateClass("nav-bar", "H")
 func GenerateClass(tag, slot string) (string, error) {
 	if !isValidCustomElementTag(tag) {
-		return "", log.E("codegen.GenerateClass", "custom element tag must be a lowercase hyphenated name: "+tag, nil)
+		message := "custom element tag must be a lowercase hyphenated name: " + tag
+		if isReservedCustomElementTag(tag) {
+			message = "custom element tag is reserved by the Web Components spec: " + tag
+		}
+		return "", log.E("codegen.GenerateClass", message, nil)
 	}
 	var b strings.Builder
 	err := wcTemplate.Execute(&b, struct {
diff --git a/codegen/codegen_test.go b/codegen/codegen_test.go
index 36df710..0eac9ed 100644
--- a/codegen/codegen_test.go
+++ b/codegen/codegen_test.go
@@ -28,6 +28,9 @@ func TestGenerateClass_Bad_InvalidTag(t *testing.T) {
 
 	_, err = GenerateClass("nav bar", "C")
 	assert.Error(t, err, "custom element names must reject spaces")
+
+	_, err = GenerateClass("annotation-xml", "C")
+	assert.Error(t, err, "reserved custom element names must be rejected")
 }
 
 func TestGenerateRegistration_Good(t *testing.T) {
@@ -96,6 +99,16 @@ func TestGenerateBundle_Bad_InvalidSlotKey(t *testing.T) {
 	assert.Contains(t, err.Error(), `"X"`)
 }
 
+func TestGenerateBundle_Bad_ReservedTag(t *testing.T) {
+	slots := map[string]string{
+		"H": "annotation-xml",
+	}
+
+	_, err := GenerateBundle(slots)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "reserved")
+}
+
 func TestGenerateTypeDefinitions_Good(t *testing.T) {
 	slots := map[string]string{
 		"H": "nav-bar",
@@ -145,3 +158,17 @@ func TestGenerateTypeDefinitions_SkipsInvalidTags(t *testing.T) {
 	assert.NotContains(t, dts, "nav bar")
 	assert.Equal(t, 1, strings.Count(dts, "extends HTMLElement"))
 }
+
+func TestGenerateTypeDefinitions_SkipsReservedTags(t *testing.T) {
+	slots := map[string]string{
+		"H": "annotation-xml",
+		"C": "nav-bar",
+	}
+
+	dts, err := GenerateTypeDefinitions(slots)
+	require.NoError(t, err)
+
+	assert.Contains(t, dts, `"nav-bar": NavBar;`)
+	assert.NotContains(t, dts, "annotation-xml")
+	assert.Equal(t, 1, strings.Count(dts, "extends HTMLElement"))
+}