Fastmail looks fine to me with no font issues in Chrome 77.0.3865.75 (64-bit).
- Your second post contained only CSP (Content Security Policy) errors which caused fonts to not load. Your issue might be caused by browser extensions.I don't see any CSP violations at the Fastmail website using Chrome 77 on a Windows 10 PC.
- Your third post shows a failure to load something from the Fastmail sandbox site (www.fastmailusercontent.com.
- That special site is used to serve external content to your browser so that someone can't send you an email and discover your IP address when it's downloaded from the external site to your browser. So Fastmail loads the content from their end (and so their IP) and then your browser gets it from that www.fastmailusercontent.com content site.
- Fastmail could just load the content every time you open a different message in the list. But to improve performance I believe they cache message content, and it appears to me that there is caching both in your browser and at the www.fastmailusercontent.com site.
- So in some cases you can select a message and it loads immediately with no errors. But if the cache entry for that message is stale or missing you can get a 404 error (not found). The cache is then refreshed and the content is sent to the browser using jmap protocol.
- So after testing this for a while (reading messages in various orders), I believe this is a cache miss transient "error". Not really a problem. It's just how cached systems work.
- More about Content Security Policy and why email content is served through a different site:
Bill