If your browser is set to automatically delete site data and cookies on close, data meant to be taken offline will be removed by the browser. This setting is disabled by default but to verify:
Google Chrome
Click the three dots to see the menu in the top right of the page
Click "Privacy and Security" on the left side
Click "Site Settings"
Click "Cookies and Site Date"
Make sure the "Clear cookies and site data when you quit Chrome" is turned off