Load CSS file after page load, which is not required for the initial view of the page.

loadStylesheet.js

const loadStyleSheet = src => {
  if(document.createStylesheet) {
    document.createStylesheet(src);
  } else {
    const stylesheet = document.createElement('link');
    stylesheet.href = src;
    stylesheet.rel = 'text/css';
    document.querySelector('head').appendChild(stylesheet);
  }
}

window.onload = function() {
  console.log('Loaded');
  const path = 'CSS FILE PATH WHICH SHOULD BE LOADED LATER';
  loadStyleSheet(path);
}