webapi: script-created documents are UTF-8; URLs use the document encoding

Fixes the last failing subtest of WPT
/dom/nodes/DOMImplementation-createHTMLDocument.html ("URL parsing"):
resolving `a.href = "http://example.org/?ä"` on an anchor inside a
createHTMLDocument() document returned "?%E4" instead of "?%C3%A4".

Two spec violations combined:

- Per DOM, documents synthesized by script (createHTMLDocument,
  createDocument, new Document()) have the UTF-8 encoding. We had no
  per-document encoding at all: document.characterSet always reflected
  the frame's charset, so the new document inherited windows-1252 from
  the test page (which uses that encoding deliberately to catch this).
  Document gains a _charset override (same pattern as _content_type),
  set to UTF-8 in DOMImplementation.createHTMLDocument/createDocument
  and the Document constructor.

- Node.resolveURL encoded the query string with the frame's charset.
  Per the URL/HTML specs the query percent-encoding uses the encoding
  of the element's node document, so it now resolves the owning
  document and uses its encoding, falling back to the frame's.

Coverage:
- /dom/nodes/DOMImplementation-createHTMLDocument.html 12/13 -> 13/13
  (fully green)
- /dom/nodes/Document-constructor.html 3/5 -> 4/5

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/browser/webapi/DOMImplementation.zig

@@ -50,6 +50,7 @@ pub fn createHTMLDocument(_: *const DOMImplementation, title: ?js.NullableString
     const document = (try frame._factory.document(Node.Document.HTMLDocument{ ._proto = undefined })).asDocument();
     document._ready_state = .complete;
     document._url = "about:blank";
+    document._charset = "UTF-8";
 
     {
         const doctype = try frame._factory.node(DocumentType{
@@ -101,6 +102,7 @@ pub fn createDocument(_: *const DOMImplementation, namespace_nullable: js.Nullab
     // Create XML Document
     const document = (try frame._factory.document(Node.Document.XMLDocument{ ._proto = undefined })).asDocument();
     document._url = "about:blank";
+    document._charset = "UTF-8";
     // Per spec the content type depends on the requested namespace.
     document._content_type = blk: {
         const ns = namespace_ orelse break :blk "application/xml";

src/browser/webapi/Document.zig

@@ -56,6 +56,9 @@ _frame: ?*Frame = null,
 _url: ?[:0]const u8 = null, // URL for documents created via DOMImplementation (about:blank)
 // content type override for documents created via DOMImplementation.createDocument
 _content_type: ?[]const u8 = null,
+// encoding override: documents synthesized by script (createHTMLDocument,
+// createDocument) are UTF-8 regardless of the frame's encoding
+_charset: ?[]const u8 = null,
 _ready_state: ReadyState = .loading,
 _current_script: ?*Element.Html.Script = null,
 _elements_by_id: std.StringHashMapUnmanaged(*Element) = .empty,
@@ -159,6 +162,14 @@ pub fn setLocation(self: *Document, url: [:0]const u8) !void {
     return frame.scheduleNavigation(url, .{ .reason = .script, .kind = .{ .push = null } }, .{ .script = frame });
 }
 
+pub fn getCharset(self: *const Document) []const u8 {
+    if (self._charset) |charset| {
+        return charset;
+    }
+    const doc_frame = self._frame orelse return "UTF-8";
+    return doc_frame.charset;
+}
+
 pub fn getContentType(self: *const Document) []const u8 {
     if (self._content_type) |content_type| {
         return content_type;
@@ -1417,6 +1428,7 @@ pub const JsApi = struct {
         return frame._factory.node(Document{
             ._proto = undefined,
             ._type = .generic,
+            ._charset = "UTF-8",
         });
     }
 
@@ -1508,8 +1520,7 @@ pub const JsApi = struct {
     pub const compatMode = bridge.property("CSS1Compat", .{ .template = false });
 
     fn getCharacterSet(self: *const Document) []const u8 {
-        const doc_frame = self._frame orelse return "UTF-8";
-        return doc_frame.charset;
+        return self.getCharset();
     }
     pub const referrer = bridge.property("", .{ .template = false });
 

src/browser/webapi/Node.zig

@@ -582,7 +582,11 @@ pub const ResolveURLOpts = struct {
 pub fn resolveURL(self: *const Node, url: anytype, frame: *Frame, opts: ResolveURLOpts) ![:0]const u8 {
     const owner_frame = self.ownerFrame(frame);
     const allocator = opts.allocator orelse frame.call_arena;
-    return URL.resolve(allocator, owner_frame.base(), url, .{ .encoding = owner_frame.charset });
+    // The owning document's encoding, not the frame's: script-created
+    // documents (e.g. createHTMLDocument) are always UTF-8.
+    const doc: ?*const Document = if (self._type == .document) self._type.document else self.ownerDocument(frame);
+    const encoding = if (doc) |d| d.getCharset() else owner_frame.charset;
+    return URL.resolve(allocator, owner_frame.base(), url, .{ .encoding = encoding });
 }
 
 // Same as `resolveURL` but can't return `TypeError`, this is needed for multiple