old_contents: import os
  
  from django.core.checks import Error, register
  
  
  @register()
  def css_install_check(app_configs, **kwargs):
      errors = []
  
      css_path = os.path.join(
          os.path.dirname(__file__), 'static', 'wagtailadmin', 'css', 'normalize.css'
      )
  
      if not os.path.isfile(css_path):
          error_hint = """
              Most likely you are running a development (non-packaged) copy of
              Wagtail and have not built the static assets -
              see http://docs.wagtail.io/en/latest/contributing/developing.html
  
              File not found: %s
          """ % css_path
  
          errors.append(
              Error(
                  "CSS for the Wagtail admin is missing",
                  hint=error_hint,
                  id='wagtailadmin.E001',
              )
          )
      return errors
  
new_contents: import os
  
  from django.core.checks import Warning, register
  
  
  @register()
  def css_install_check(app_configs, **kwargs):
      errors = []
  
      css_path = os.path.join(
          os.path.dirname(__file__), 'static', 'wagtailadmin', 'css', 'normalize.css'
      )
  
      if not os.path.isfile(css_path):
          error_hint = """
              Most likely you are running a development (non-packaged) copy of
              Wagtail and have not built the static assets -
              see http://docs.wagtail.io/en/latest/contributing/developing.html
  
              File not found: %s
          """ % css_path
  
          errors.append(
              Warning(
                  "CSS for the Wagtail admin is missing",
                  hint=error_hint,
                  id='wagtailadmin.W001',
              )
          )
      return errors
  
subject: Throw warning on missing CSS rather than error, so that tests can still run on Dj1.7