Can I access web data when the app user is online and use these data in my local web assets?

Yeah! 😉 You would insert an external script call like

<script type="text/javascript" src="YOUR DOMAIN HERE/external.js"></script>

This would be called automatically if the user is online (and will be ignored if the user is offline).

Then, you can do ANYTHING like updating localStorage items:

https://stackoverflow.com/questions/37052087/how-do-i-update-localstorage-items