7230 says:

The trailer fields are identical to header fields, except they are sent in a chunked trailer instead of the message's header section.

and:

When a chunked message containing a non-empty trailer is received, the recipient MAY process the fields (aside from those forbidden above) as if they were appended to the message's header section.

As discussed at the 2016 HTTP Workshop as part of @annevk's Fetch presentation, this isn't necessarily sensible.