diff --git a/src/textual/highlight.py b/src/textual/highlight.py
index fc0ed33abf..2b899f9237 100644
--- a/src/textual/highlight.py
+++ b/src/textual/highlight.py
@@ -52,7 +52,7 @@ class HighlightTheme:
     }
 
 
-def guess_language(code: str, path: str) -> str:
+def guess_language(code: str, path: str | None) -> str:
     """Guess the language based on the code and path.
     The result may be used in the [highlight][textual.highlight.highlight] function.
 
@@ -64,19 +64,28 @@ def guess_language(code: str, path: str) -> str:
         The language, suitable for use with Pygments.
     """
 
-    if path is not None and os.path.splitext(path)[-1] == ".tcss":
+    if path and os.path.splitext(path)[-1] == ".tcss":
         # A special case for TCSS files which aren't known outside of Textual
         return "scss"
 
     lexer: Lexer | None = None
     lexer_name = "default"
     if code:
-        try:
-            lexer = guess_lexer_for_filename(path, code)
-        except ClassNotFound:
-            pass
+        if path:
+            try:
+                lexer = guess_lexer_for_filename(path, code)
+            except ClassNotFound:
+                pass
+
+        if lexer is None:
+            from pygments.lexers import guess_lexer
+
+            try:
+                lexer = guess_lexer(code)
+            except Exception:
+                pass
 
-    if not lexer:
+    if not lexer and path:
         try:
             _, ext = os.path.splitext(path)
             if ext:
@@ -113,9 +122,7 @@ def highlight(
     Returns:
         A Content instance which may be used in a widget.
     """
-    if language is None:
-        if path is None:
-            raise RuntimeError("One of 'language' or 'path' must be supplied.")
+    if not language:
         language = guess_language(code, path)
 
     assert language is not None
diff --git a/src/textual/widgets/_markdown.py b/src/textual/widgets/_markdown.py
index 397b265b50..786dd67403 100644
--- a/src/textual/widgets/_markdown.py
+++ b/src/textual/widgets/_markdown.py
@@ -870,12 +870,13 @@ def __init__(self, markdown: Markdown, token: Token, code: str) -> None:
         self.lexer = token.info
         self._highlighted_code = self.highlight(self.code, self.lexer)
 
+    @property
     def allow_horizontal_scroll(self) -> bool:
         return True
 
     @classmethod
     def highlight(cls, code: str, language: str) -> Content:
-        return highlight(code, language=language)
+        return highlight(code, language=language or None)
 
     def _copy_context(self, block: MarkdownBlock) -> None:
         if isinstance(block, MarkdownFence):
diff --git a/tests/test_highlight.py b/tests/test_highlight.py
index 0178835221..a2a116860a 100644
--- a/tests/test_highlight.py
+++ b/tests/test_highlight.py
@@ -19,9 +19,13 @@ def test_highlight() -> None:
 @pytest.mark.parametrize(
     "code,path,language",
     [
+        ("", "", "default"),
+        ("# Don't matter", "foo.tcss", "scss"),
         ("import this", "foo.py", "python"),
         ("<xml>", "foo.xml", "xml"),
         ("{}", "data.json", "json"),
+        ("#! python", "", "python"),
+        ("", "foo.py", "python"),
     ],
 )
 def test_guess_language(code: str, path: str, language: str) -> None: