Don't reject a document that starts with a "<?xml…" processing instruction

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');
     }

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,
         });

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');