fix: follow spec for customElement option

.changeset/stale-rats-check.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: follow spec for `customElement` option

packages/svelte/messages/compile-errors/template.md

@@ -344,7 +344,15 @@ HTML restricts where certain elements can appear. In case of a violation the bro
 
 ## svelte_options_invalid_tagname
 
-> Tag name must be two or more words joined by the "-" character
+> Tag name must be lowercase and hyphenated
+
+See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
+
+## svelte_options_reserved_tagname
+
+> Tag name is reserved
+
+See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
 
 ## svelte_options_unknown_attribute
 

packages/svelte/src/compiler/errors.js

@@ -1352,12 +1352,21 @@ export function svelte_options_invalid_customelement_shadow(node) {
 }
 
 /**
- * Tag name must be two or more words joined by the "-" character
+ * Tag name must be lowercase and hyphenated
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function svelte_options_invalid_tagname(node) {
-	e(node, "svelte_options_invalid_tagname", "Tag name must be two or more words joined by the \"-\" character");
+	e(node, "svelte_options_invalid_tagname", "Tag name must be lowercase and hyphenated");
+}
+
+/**
+ * Tag name is reserved
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function svelte_options_reserved_tagname(node) {
+	e(node, "svelte_options_reserved_tagname", "Tag name is reserved");
 }
 
 /**

packages/svelte/src/compiler/phases/1-parse/read/options.js

@@ -3,8 +3,6 @@
 import { NAMESPACE_MATHML, NAMESPACE_SVG } from '../../../../constants.js';
 import * as e from '../../../errors.js';
 
-const regex_valid_tag_name = /^[a-zA-Z][a-zA-Z0-9]*-[a-zA-Z0-9-]+$/;
-
 /**
  * @param {AST.SvelteOptionsRaw} node
  * @returns {AST.Root['options']}
@@ -229,6 +227,21 @@ function get_boolean_value(attribute) {
 	return value;
 }
 
+// https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
+const tag_name_char =
+	'[a-z0-9_.\xB7\xC0-\xD6\xD8-\xF6\xF8-\u037D\u037F-\u1FFF\u200C-\u200D\u203F-\u2040\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD\u{10000}-\u{EFFFF}-]';
+const regex_valid_tag_name = new RegExp(`^[a-z]${tag_name_char}*-${tag_name_char}*$`, 'u');
+const reserved_tag_names = [
+	'annotation-xml',
+	'color-profile',
+	'font-face',
+	'font-face-src',
+	'font-face-uri',
+	'font-face-format',
+	'font-face-name',
+	'missing-glyph'
+];
+
 /**
  * @param {any} attribute
  * @param {string | null} tag
@@ -238,7 +251,11 @@ function validate_tag(attribute, tag) {
 	if (typeof tag !== 'string') {
 		e.svelte_options_invalid_tagname(attribute);
 	}
-	if (tag && !regex_valid_tag_name.test(tag)) {
-		e.svelte_options_invalid_tagname(attribute);
+	if (tag) {
+		if (!regex_valid_tag_name.test(tag)) {
+			e.svelte_options_invalid_tagname(attribute);
+		} else if (reserved_tag_names.includes(tag)) {
+			e.svelte_options_reserved_tagname(attribute);
+		}
 	}
 }

packages/svelte/tests/validator/samples/tag-emoji/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		customElement: true
+	}
+});

packages/svelte/tests/validator/samples/tag-emoji/input.svelte

@@ -0,0 +1 @@
+<svelte:options customElement="emotion-😍" />

packages/svelte/tests/validator/samples/tag-hyphen/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		customElement: true
+	}
+});

packages/svelte/tests/validator/samples/tag-hyphen/input.svelte

@@ -0,0 +1 @@
+<svelte:options customElement="custom-" />

packages/svelte/tests/validator/samples/tag-invalid/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "svelte_options_invalid_tagname",
-		"message": "Tag name must be two or more words joined by the \"-\" character",
+		"message": "Tag name must be lowercase and hyphenated",
 		"start": {
 			"line": 1,
 			"column": 16

packages/svelte/tests/validator/samples/tag-reserved/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "svelte_options_reserved_tagname",
+		"message": "Tag name is reserved",
+		"start": {
+			"line": 1,
+			"column": 16
+		},
+		"end": {
+			"line": 1,
+			"column": 41
+		}
+	}
+]

packages/svelte/tests/validator/samples/tag-reserved/input.svelte

@@ -0,0 +1 @@
+<svelte:options customElement="font-face" />