fix: strip XML prolog from the text/html fallback body

tests/test_site.py

@@ -115,6 +115,26 @@ class TestWmlCardUrls:
         assert b"{.card" not in doc.xhtml
 
 
+class TestHtmlFallbackHasNoXmlProlog:
+    def test_html_field_has_no_prolog(self, site: Site):
+        # A browser parsing this as text/html (rather than XML) tolerates
+        # a leading <?xml ...?> as a bogus comment and still renders the
+        # page, but logs a console warning -- so the text/html fallback
+        # must not carry it.
+        doc = site.resolve("/")
+        assert not doc.html.startswith(b"<?xml")
+        assert doc.html.startswith(b"<!DOCTYPE html")
+
+    def test_xhtml_field_keeps_the_prolog(self, site: Site):
+        # Real XHTML-MP/WAP 2.0 clients still need well-formed XML.
+        doc = site.resolve("/")
+        assert doc.xhtml.startswith(b'<?xml version="1.0"')
+
+    def test_html_and_xhtml_are_otherwise_identical(self, site: Site):
+        doc = site.resolve("/")
+        assert doc.html == doc.xhtml[doc.xhtml.index(b"<!DOCTYPE"):]
+
+
 class TestLiveReload:
     def test_edited_file_is_rerendered(self, tmp_path: Path):
         target = tmp_path / "index.md"