Short answer
HubSpot's KB: the web version is a page generated from the email, linked in its body and not indexed by search engines. Only contact and company tokens render; others show fallbacks. On Marketing Hub Starter it expires a year after publishing.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Set good fallbacks for any other tokens.
3. Related
Dark mode and plain text.