The Event - Details content block functions as the main content for an event invitation email. It has space for all of the typical event details the user expects: the name of the event, date, time, location, and a link to more information.
There are some variants of this block, including a version with a button CTA and a text-based CTA. The version with the button gives higher prominence to the link out, and should be used in situations where you want the user to RSVP. The text-based link is better when there may be additional details about the event below the event details content block that you'd also like the recipient to read.
The styling is the same between formal and informal.