Ignore warnings encoded as syntax highlighting

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

agda.rb

@@ -46,7 +46,17 @@ class AgdaContext
           # assumes that links can't span multiple pages, and that links
           # aren't nested, so ensure that the parent of the textual node
           # is the preformatted block itself.
-          raise "unsupported Agda HTML output" if at.parent.name != "pre"
+          if at.parent.name != "pre"
+            # Costmetic highlight warnings are sometimes applied to newlines.
+            # If they don't have content, treat them as normal newlines at the
+            # top level.
+            #
+            # This is an <a class="CosmeticProblem">\n</a> node.
+            unless at.parent.name == "a" and at.parent['class'] == "CosmeticProblem" and at.content.strip.empty?
+              raise "unsupported Agda HTML output in file #{file} at line #{line} (content #{at.content.inspect})"
+            end
+          end
+
 
           # Increase the line and track the final offset. Written as a loop
           # in case we eventually want to add some handling for the pieces