Node:Content type specification, Next:Client-side name expansion, Previous:Text formatting, Up:Client Conventions
This convention is understood by all popular clients. If the first line
is one of a few predefined strings, then this string specifies the type
of text. Currently only the strings "html:" and "enriched:" are
supported, specifying text/html and text/enriched respectively.
Starting with protocol version 10, this ugly workaround is obsolete. Use aux-items to specify content type instead.