code: def buildhtmlheader(self):
          
          #Highcharts lib/ needs to make sure it's up to date
          
          if self.drilldown_flag:
              self.add_JSsource('https://code.highcharts.com/maps/modules/drilldown.js')
  
          self.header_css = [
              '<link href="%s" rel="stylesheet" />' % h for h in self.CSSsource
          ]
  
          self.header_js = [
              '<script type="text/javascript" src="%s"></script>' % h for h in self.JSsource
          ]
  
          self.htmlheader = ''
          for css in self.header_css:
              self.htmlheader += css
          for js in self.header_js:
              self.htmlheader += js
docs: generate HTML header content