From 779dc272210b1e5cc8fcf36ebfb8dee58ba6629a Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Thu, 23 Jul 2026 18:37:57 +0100
Subject: [PATCH] =?UTF-8?q?Don't=20reject=20a=20document=20that=20starts?=
 =?UTF-8?q?=20with=20a=20"<=3Fxml=E2=80=A6"=20processing=20instruction?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

`consumeXmlDeclaration` matched the 5-character literal `<?xml` and then
required whitespace, so a document beginning with a processing instruction
whose target merely starts with `xml` — `<?xml-stylesheet ?>`,
`<?xml-model ?>` — was forced down the XML-declaration path and rejected
with "Invalid XML declaration":

  parseXml('<?xml-stylesheet type="text/xsl" href="a.xsl"?><a/>') // threw

Those are valid documents: an XML declaration is `'<?xml' S 'version' …`
(the `<?xml` must be followed by whitespace), whereas `xml-stylesheet` and
`xml-model` are distinct, well-formed PITargets, and `prolog ::= XMLDecl?
Misc*` (XML 1.0 §2.8) allows a processing instruction as the first item
when no declaration is present.

After consuming `<?xml`, if the next character is a name character this is
such a PI: rewind and let it be parsed as a processing instruction. `<?xml`
followed by whitespace (a real declaration) or by a non-name character
(e.g. `<?xml?>`, whose bare `xml` target is reserved) is unchanged.
---
 src/lib/Parser.ts                          | 9 +++++++++
 tests/lib/Parser.test.js                   | 6 +++---
 tests/lib/XmlProcessingInstruction.test.js | 7 +++++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/lib/Parser.ts b/src/lib/Parser.ts
index 5df9805..82cd65f 100644
--- a/src/lib/Parser.ts
+++ b/src/lib/Parser.ts
@@ -712,6 +712,15 @@ export class Parser {
       return false;
     }
 
+    // `<?xml` begins an XML declaration only when it's immediately followed by
+    // whitespace. If the next character is a name character, this is a
+    // processing instruction whose target merely begins with "xml" (e.g.
+    // `<?xml-stylesheet ... ?>`); rewind and let it be parsed as a PI.
+    if (syntax.isNameChar(scanner.peek())) {
+      scanner.reset(startIndex);
+      return false;
+    }
+
     if (!this.consumeWhitespace()) {
       throw this.error('Invalid XML declaration');
     }
diff --git a/tests/lib/Parser.test.js b/tests/lib/Parser.test.js
index 0eca1a2..b443efb 100644
--- a/tests/lib/Parser.test.js
+++ b/tests/lib/Parser.test.js
@@ -178,13 +178,13 @@ describe('Parser', () => {
     describe('invalid XML declaration', () => {
       it('throws an error', () => {
         assert.throws(() => {
-          parseXml('<?xmlblah');
+          parseXml('<?xml?>');
         }, {
           column: 6,
-          excerpt: '<?xmlblah',
+          excerpt: '<?xml?>',
           line: 1,
           message: 'Invalid XML declaration (line 1, column 6)\n' +
-            '  <?xmlblah\n' +
+            '  <?xml?>\n' +
             '       ^\n',
           pos: 5,
         });
diff --git a/tests/lib/XmlProcessingInstruction.test.js b/tests/lib/XmlProcessingInstruction.test.js
index ab8a24a..0394d77 100644
--- a/tests/lib/XmlProcessingInstruction.test.js
+++ b/tests/lib/XmlProcessingInstruction.test.js
@@ -16,6 +16,13 @@ describe('XmlProcessingInstruction', () => {
     assert.strictEqual(JSON.stringify(root.children[0]), `{"type":"pi","name":"xml-stylesheet","content":"type=\\"text/xsl\\" href=\\"style.xsl\\""}`);
   });
 
+  it('may begin the document when its target starts with "xml"', () => {
+    let doc = parseXml('<?xml-stylesheet type="text/xsl" href="style.xsl"?><root/>');
+    assert(doc.children[0] instanceof XmlProcessingInstruction);
+    assert.strictEqual(doc.children[0].name, 'xml-stylesheet');
+    assert.strictEqual(doc.root.name, 'root');
+  });
+
   describe('constructor', () => {
     it('defaults the value of `content` to an empty string if not provided', () => {
       let pi = new XmlProcessingInstruction('foo');