gh-95953: Add a css class to changed lines of difflib.HtmlDiff make_table (#139226)

Lib/difflib.py

@@ -1924,8 +1924,11 @@ def _format_line(self,side,flag,linenum,text):
         # make space non-breakable so they don't get compressed or line wrapped
         text = text.replace(' ',' ').rstrip()
 
-        return '<td class="diff_header"%s>%s</td><td nowrap="nowrap">%s</td>' \
-               % (id,linenum,text)
+        # add a class to the td tag if there is a difference on the line
+        css_class = ' class="diff_changed" ' if flag else ' '
+
+        return f'<td class="diff_header"{id}>{linenum}</td>' \
+            + f'<td{css_class}nowrap="nowrap">{text}</td>'
 
     def _make_prefix(self):
         """Create unique anchor prefixes"""