fix(html): reserve responsive variant attribute

Co-Authored-By: Virgil <virgil@lethean.io>

responsive.go

@@ -212,6 +212,9 @@ func (r *Responsive) renderWithPath(ctx *Context, path string) string {
 			keys := slices.Collect(maps.Keys(r.attrs))
 			slices.Sort(keys)
 			for _, key := range keys {
+				if key == "data-variant" {
+					continue
+				}
 				b.WriteByte(' ')
 				b.WriteString(escapeHTML(key))
 				b.WriteString(`="`)

responsive_test.go

@@ -158,6 +158,26 @@ func TestResponsive_Attributes(t *testing.T) {
 	}
 }
 
+func TestResponsive_ReservedVariantAttributeIsIgnored(t *testing.T) {
+	ctx := NewContext()
+	r := Attr(NewResponsive().
+		Variant("desktop", NewLayout("C").C(Raw("main"))),
+		"data-variant", "override",
+	)
+
+	got := r.Render(ctx)
+
+	if count := strings.Count(got, `data-variant=`); count != 1 {
+		t.Fatalf("responsive wrapper should emit exactly one data-variant attribute, got %d in:\n%s", count, got)
+	}
+	if !strings.Contains(got, `data-variant="desktop"`) {
+		t.Fatalf("responsive wrapper should preserve its own variant name, got:\n%s", got)
+	}
+	if strings.Contains(got, `data-variant="override"`) {
+		t.Fatalf("responsive wrapper should ignore reserved data-variant attrs, got:\n%s", got)
+	}
+}
+
 func TestVariantSelector(t *testing.T) {
 	tests := []struct {
 		name    string