code: def apply_pre_filters(instance, html):
      
      # Allow pre processing. Typical use-case is HTML syntax correction.
      for post_func in appsettings.PRE_FILTER_FUNCTIONS:
          html = post_func(instance, html)
  
      return html
docs: Perform optimizations in the HTML source code.
  
      :type instance: fluent_contents.models.ContentItem
      :raise ValidationError: when one of the filters detects a problem.