doGet

function doGet() {
  return HtmlService.createTemplateFromFile('index')
  .evaluate()
  .setTitle('createPDF')
  .addMetaTag('viewport', 'width=device-width, initial-scale=1')
  .setXFrameOptionsMode(HtmlService.XFrameOptionsMode.ALLOWALL)
}

<?!= HtmlService.createHtmlOutputFromFile('css').getContent() ?>