Spoiler messages are described in MSC2010 though the MSC is unclear if the fallback is required to be sent by clients.
The fallback for spoiler messages is optional, though recommended to be sent by clients. Clients
should make reasonable efforts to represent the spoiler in the body field of a message.
The recommended fallback format is unchanged.
Additionally, this proposal opens up spoilers to any HTML-supporting message types. Currently
this includes m.text (already included by MSC2010), m.notice, and m.emote.
Clients could inadvertently spoil parts of a message by not representing the spoiler correctly
in the body of the message. The author believes this would quickly show up as a bug report
on the client due to the nature of spoilers.