Convert your emails to PDF format on offline mode (without a server interaction; secure and private) to backup your emails
This works on Vivaldi (chrome based) and the red button is with images, blue plain text - Amazing at mass downloading gmail emails selecting all and having them all process onto the desktop or folder
I compared two extensions on an Edge browser, and I like this one better. No extra setup needed like creating an account, it simply allows you to save the selected emails as PDFs into your Downloads folder. The font styles are different, but it works fine for what I needed it for.
Just used the extension now - worked great for me
https://www.save-emails-as-pdf.com/
https://olivernassar.com
https://www.save-emails-to-dropbox.com/
https://save-emails-to-google-drive.com
Joey
https://www.sort-gmail-inbox.com/
https://screenshot-tool.com
https://www.gmail-web-clipper.com/
https://sendpulse.com
https://www.emails-to-sheets.com/
https://sites.google.com/view/email-to-pdf/
https://www.emails-to-docs.com/
