accept DOCTYPE element in any case

src/generators/server-side-rendering/visitors/Element.js

@@ -1,5 +1,5 @@
 import Component from './Component.js';
-import voidElementNames from '../../../utils/voidElementNames.js';
+import isVoidElementName from '../../../utils/isVoidElementName.js';
 
 export default {
 	enter ( generator, node ) {
@@ -44,7 +44,7 @@ export default {
 			return;
 		}
 
-		if ( !voidElementNames.test( node.name ) ) {
+		if ( !isVoidElementName( node.name ) ) {
 			generator.append( `</${node.name}>` );
 		}
 	}

src/parse/state/tag.js

@@ -4,7 +4,7 @@ import readStyle from '../read/style.js';
 import { readEventHandlerDirective, readBindingDirective } from '../read/directives.js';
 import { trimStart, trimEnd } from '../utils/trim.js';
 import { decodeCharacterReferences } from '../utils/html.js';
-import voidElementNames from '../../utils/voidElementNames.js';
+import isVoidElementName from '../../utils/isVoidElementName.js';
 
 const validTagName = /^\!?[a-zA-Z]{1,}:?[a-zA-Z0-9\-]*/;
 const invalidUnquotedAttributeCharacters = /[\s"'=<>\/`]/;
@@ -84,7 +84,7 @@ export default function tag ( parser ) {
 	parser.allowWhitespace();
 
 	if ( isClosingTag ) {
-		if ( voidElementNames.test( name ) ) {
+		if ( isVoidElementName( name ) ) {
 			parser.error( `<${name}> is a void element and cannot have children, or a closing tag`, start );
 		}
 
@@ -155,7 +155,7 @@ export default function tag ( parser ) {
 
 	parser.current().children.push( element );
 
-	const selfClosing = parser.eat( '/' ) || voidElementNames.test( name );
+	const selfClosing = parser.eat( '/' ) || isVoidElementName( name );
 
 	parser.eat( '>', true );
 

src/utils/isVoidElementName.js

@@ -0,0 +1,5 @@
+const voidElementNames = /^(?:area|base|br|col|command|embed|hr|img|input|keygen|link|meta|param|source|track|wbr)$/;
+
+export default function isVoidElementName ( name ) {
+	return voidElementNames.test( name ) || name.toLowerCase() === '!doctype';
+}

src/utils/voidElementNames.js

@@ -1 +0,0 @@
-export default /^(?:area|base|br|col|command|\!doctype|embed|hr|img|input|keygen|link|meta|param|source|track|wbr)$/;