xml: validate xml declaration

And now, because we do this, we no longer have to have the parser skip anything
that looks like a ProcessingInstruction.

src/browser/frame/parse.zig

@@ -123,13 +123,5 @@ pub fn xmlDocument(frame: *Frame, xml: []const u8) !?*Document.XMLDocument {
     if (parser.err != null or parser.xml_error or doc_node.firstChild() == null) {
         return null;
     }
-
-    // If first node is a `ProcessingInstruction` (e.g. the <?xml?>
-    // declaration), skip it.
-    const first_child = doc_node.firstChild().?;
-    if (first_child.getNodeType() == 7) {
-        _ = try doc_node.removeChild(first_child, frame);
-    }
-
     return doc;
 }

src/browser/tests/domparser.html

@@ -521,6 +521,11 @@
     '<a>&#0;</a>',
     '<a><?1x?></a>',             // invalid processing instruction target
     '<?1x?><a/>',
+    '<?xml version="2.0"?><a/>', // only 1.x versions are accepted
+    '<?xml version="10.0"?><a/>',
+    '<?xml version="1."?><a/>',
+    '<?xml version=1.0?><a/>',
+    '<?xml encoding="UTF-8"?><a/>', // version is required
   ]) {
     testing.expectEqual(bad + ' -> error', bad + (isError(p.parseFromString(bad, 'text/xml')) ? ' -> error' : ' -> ok'));
   }
@@ -529,6 +534,8 @@
   for (const good of [
     '<a/>',
     '<?xml version="1.0" encoding="UTF-8"?>\n<a/>\n',
+    "<?xml version = '1.1075' ?><a/>",
+    '<?xml version="1.000" standalone="yes"?><a/>',
     '\uFEFF<a/>',
     '<a><![CDATA[<x>]]><!-- c --><?pi d?></a><!-- trailing -->',
     '<a b="x&quot;y&apos;&lt;&#169;"/>',
@@ -548,6 +555,12 @@
   testing.expectEqual('svg', svg.doctype.name);
   testing.expectEqual('-//W3C//DTD SVG 1.1//EN', svg.doctype.publicId);
   testing.expectEqual('svg', svg.documentElement.localName);
+
+  // the XML declaration creates no node, a leading processing instruction does
+  const decl = p.parseFromString('<?xml version="1.0"?><?xml-stylesheet href="a.css"?><a/>', 'text/xml');
+  testing.expectEqual(2, decl.childNodes.length);
+  testing.expectEqual('xml-stylesheet', decl.firstChild.target);
+  testing.expectEqual('xml-stylesheet', p.parseFromString('<?xml-stylesheet href="a.css"?><a/>', 'text/xml').firstChild.target);
 }
 </script>
 

src/rust/html5ever/lib.rs

@@ -874,6 +874,17 @@ impl<'arena> xml5ever::tokenizer::TokenSink for UnclosedTagSink<'arena> {
                         .parse_error(std::borrow::Cow::Borrowed("Unclosed element at EOF"));
                 }
             }
+            // The XML declaration isn't a processing instruction, so no node
+            // is created for it. xml5ever doesn't validate it.
+            Token::ProcessingInstruction(pi) if &*pi.target == "xml" => {
+                if !is_valid_xml_declaration(&pi.data) {
+                    use xml5ever::tree_builder::TreeSink;
+                    self.tb
+                        .sink
+                        .parse_error(std::borrow::Cow::Borrowed("Invalid XML declaration"));
+                }
+                return xml5ever::tokenizer::ProcessResult::Continue;
+            }
             _ => {}
         }
         self.tb.process_token(token)
@@ -884,6 +895,30 @@ impl<'arena> xml5ever::tokenizer::TokenSink for UnclosedTagSink<'arena> {
     }
 }
 
+// The declaration must start with `version="1.x"`: browsers accept any 1.x
+// (XML 1.0 5th edition's VersionNum is `1.[0-9]+`) and reject everything else.
+fn is_valid_xml_declaration(data: &str) -> bool {
+    fn trim(s: &str) -> &str {
+        s.trim_start_matches([' ', '\t', '\r', '\n'])
+    }
+    let Some(rest) = trim(data).strip_prefix("version").map(trim) else {
+        return false;
+    };
+    let Some(rest) = rest.strip_prefix('=').map(trim) else {
+        return false;
+    };
+    let Some(quote) = rest.chars().next().filter(|c| *c == '"' || *c == '\'') else {
+        return false;
+    };
+    let Some((version, _)) = rest[1..].split_once(quote) else {
+        return false;
+    };
+    match version.strip_prefix("1.") {
+        Some(minor) => !minor.is_empty() && minor.bytes().all(|b| b.is_ascii_digit()),
+        None => false,
+    }
+}
+
 // xml5ever::driver::XmlParser, minus the tree-builder-typed tokenizer so the
 // UnclosedTagSink can sit in between.
 struct XmlDocumentParser<'arena> {