diff -urN mako/lexer.py mako.new/lexer.py
--- mako/lexer.py	2021-01-15 02:28:51.000000000 +0800
+++ mako.new/lexer.py	2022-09-22 11:19:51.277358075 +0800
@@ -295,20 +295,24 @@
         return self.template
 
     def match_tag_start(self):
-        match = self.match(
-            r"""
+        reg = r"""
             \<%     # opening tag
 
             ([\w\.\:]+)   # keyword
 
-            ((?:\s+\w+|\s*=\s*|".*?"|'.*?')*)  # attrname, = \
-                                               #        sign, string expression
+            ((?:\s+\w+|\s*=\s*|"[^"]*?"|'[^']*?'|\s*,\s*)*)  # attrname, = \
+                                     #        sign, string expression
+                                     # comma is for backwards compat
+                                     # identified in #366
 
             \s*     # more whitespace
 
             (/)?>   # closing
 
-            """,
+        """
+
+        match = self.match(
+            reg,
             re.I | re.S | re.X,
         )