I often access my Exchange Online mailbox via my browser when working from home rather than firing up the Outlook client on the work laptop and recently had cause to add an appointment that was in an ICS file to my calendar.
Now this didn’t used to be possible via OWA but I’ve discovered it now is, by going New > Add calendar > from file:
then browsing to the file in question
and then clicking on save, and low and behold, the appointments in my calendar, no full fat client required!
Not sure how long the feature has been there for but I’m glad it has been added.