To be really XML-safe, don't emit XHTML+MathML named entities. (Ported MathML::Entities to Ruby.)

app/controllers/wiki_controller.rb

@@ -294,7 +294,7 @@ class WikiController < ApplicationController
 
   def s5
     if @web.markup == :markdownMML or @web.markup == :markdown
-      @s5_content = sanitize_html(Maruku.new(@page.content.delete("\r\x01-\x08\x0B\x0C\x0E-\x1F"),
+      @s5_content = sanitize_html(Maruku.new(@page.content.delete("\r\x01-\x08\x0B\x0C\x0E-\x1F").to_ncr,
            {:math_enabled => true, :math_numbered => ['\\[','\\begin{equation}'], :content_only => true,
             :author => @page.author, :title => @page.plain_name}).to_s5)
     end

lib/chunks/engines.rb

@@ -43,7 +43,7 @@ module Engines
       require_dependency 'maruku'
       require_dependency 'maruku/ext/math'
       html = Maruku.new(@content.delete("\r\x01-\x08\x0B\x0C\x0E-\x1F"), {:math_enabled => false}).to_html
-      sanitize_html(html)
+      sanitize_html(html).to_ncr
     end
   end
 
@@ -55,7 +55,7 @@ module Engines
       require_dependency 'maruku/ext/math'
       html = Maruku.new(@content.delete("\r\x01-\x08\x0B\x0C\x0E-\x1F"),
             {:math_enabled => true, :math_numbered => ['\\[','\\begin{equation}']}).to_html
-      sanitize_html(html)
+      sanitize_html(html).to_ncr
     end
   end