diff --git a/codegen/codegen.go b/codegen/codegen.go
new file mode 100644
index 0000000..6e60783
--- /dev/null
+++ b/codegen/codegen.go
@@ -0,0 +1,90 @@
+package codegen
+
+import (
+	"fmt"
+	"strings"
+	"text/template"
+)
+
+// wcTemplate is the Web Component class template.
+// Uses closed Shadow DOM for isolation. Content is set via the shadow root's
+// DOM API using trusted go-html codegen output (never user input).
+var wcTemplate = template.Must(template.New("wc").Parse(`class {{.ClassName}} extends HTMLElement {
+  #shadow;
+  constructor() {
+    super();
+    this.#shadow = this.attachShadow({ mode: "closed" });
+  }
+  connectedCallback() {
+    this.#shadow.textContent = "";
+    const slot = this.getAttribute("data-slot") || "{{.Slot}}";
+    this.dispatchEvent(new CustomEvent("wc-ready", { detail: { tag: "{{.Tag}}", slot } }));
+  }
+  render(html) {
+    const tpl = document.createElement("template");
+    tpl.insertAdjacentHTML("afterbegin", html);
+    this.#shadow.textContent = "";
+    this.#shadow.appendChild(tpl.content.cloneNode(true));
+  }
+}`))
+
+// GenerateClass produces a JS class definition for a custom element.
+func GenerateClass(tag, slot string) (string, error) {
+	if !strings.Contains(tag, "-") {
+		return "", fmt.Errorf("codegen: custom element tag %q must contain a hyphen", tag)
+	}
+	var b strings.Builder
+	err := wcTemplate.Execute(&b, struct {
+		ClassName, Tag, Slot string
+	}{
+		ClassName: TagToClassName(tag),
+		Tag:       tag,
+		Slot:      slot,
+	})
+	if err != nil {
+		return "", fmt.Errorf("codegen: template exec: %w", err)
+	}
+	return b.String(), nil
+}
+
+// GenerateRegistration produces the customElements.define() call.
+func GenerateRegistration(tag, className string) string {
+	return fmt.Sprintf(`customElements.define("%s", %s);`, tag, className)
+}
+
+// TagToClassName converts a kebab-case tag to PascalCase class name.
+func TagToClassName(tag string) string {
+	parts := strings.Split(tag, "-")
+	var b strings.Builder
+	for _, p := range parts {
+		if len(p) > 0 {
+			b.WriteString(strings.ToUpper(p[:1]))
+			b.WriteString(p[1:])
+		}
+	}
+	return b.String()
+}
+
+// GenerateBundle produces all WC class definitions and registrations
+// for a set of HLCRF slot assignments.
+func GenerateBundle(slots map[string]string) (string, error) {
+	seen := make(map[string]bool)
+	var b strings.Builder
+
+	for slot, tag := range slots {
+		if seen[tag] {
+			continue
+		}
+		seen[tag] = true
+
+		cls, err := GenerateClass(tag, slot)
+		if err != nil {
+			return "", err
+		}
+		b.WriteString(cls)
+		b.WriteByte('\n')
+		b.WriteString(GenerateRegistration(tag, TagToClassName(tag)))
+		b.WriteByte('\n')
+	}
+	return b.String(), nil
+}
diff --git a/codegen/codegen_test.go b/codegen/codegen_test.go
new file mode 100644
index 0000000..28a6aa2
--- /dev/null
+++ b/codegen/codegen_test.go
@@ -0,0 +1,54 @@
+package codegen
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateClass_Good(t *testing.T) {
+	js, err := GenerateClass("photo-grid", "C")
+	require.NoError(t, err)
+	assert.Contains(t, js, "class PhotoGrid extends HTMLElement")
+	assert.Contains(t, js, "attachShadow")
+	assert.Contains(t, js, `mode: "closed"`)
+	assert.Contains(t, js, "photo-grid")
+}
+
+func TestGenerateClass_Bad_InvalidTag(t *testing.T) {
+	_, err := GenerateClass("invalid", "C")
+	assert.Error(t, err, "custom element names must contain a hyphen")
+}
+
+func TestGenerateRegistration_Good(t *testing.T) {
+	js := GenerateRegistration("photo-grid", "PhotoGrid")
+	assert.Contains(t, js, "customElements.define")
+	assert.Contains(t, js, `"photo-grid"`)
+	assert.Contains(t, js, "PhotoGrid")
+}
+
+func TestTagToClassName_Good(t *testing.T) {
+	tests := []struct{ tag, want string }{
+		{"photo-grid", "PhotoGrid"},
+		{"nav-breadcrumb", "NavBreadcrumb"},
+		{"my-super-widget", "MySuperWidget"},
+	}
+	for _, tt := range tests {
+		got := TagToClassName(tt.tag)
+		assert.Equal(t, tt.want, got, "TagToClassName(%q)", tt.tag)
+	}
+}
+
+func TestGenerateBundle_Good(t *testing.T) {
+	slots := map[string]string{
+		"H": "nav-bar",
+		"C": "main-content",
+	}
+	js, err := GenerateBundle(slots)
+	require.NoError(t, err)
+	assert.Contains(t, js, "NavBar")
+	assert.Contains(t, js, "MainContent")
+	assert.Equal(t, 2, strings.Count(js, "extends HTMLElement"))
+}
diff --git a/go.mod b/go.mod
index 01fbda6..e6d771e 100644
--- a/go.mod
+++ b/go.mod
@@ -4,6 +4,14 @@ go 1.25.5
 
 replace forge.lthn.ai/core/go-i18n => ../go-i18n
 
-require forge.lthn.ai/core/go-i18n v0.0.0-00010101000000-000000000000
+require (
+	forge.lthn.ai/core/go-i18n v0.0.0-00010101000000-000000000000
+	github.com/stretchr/testify v1.11.1
+)
 
-require golang.org/x/text v0.33.0 // indirect
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	golang.org/x/text v0.33.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
index 62a1313..656be30 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,12 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 golang.org/x/text v0.33.0 h1:B3njUFyqtHDUI5jMn1YIr5B0IE2U0qck04r6d4KPAxE=
 golang.org/x/text v0.33.0/go.mod h1:LuMebE6+rBincTi9+xWTY8TztLzKHc/9C1uBCG27+q8=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=