Re: Format Negociation in Practice [Was: Versioning HTML at the server]

Earl Hood (
Tue, 18 Oct 1994 22:44:03 +0100

> I don't know if this means more tutorial documentation, or some
> http server configuration options, or changes to clients, or what.
> I have a few ideas, but I'd like to hear from other folks: how can
> we clean this mess up? Or is format negociation just not gonna work?

Here's somethings off the top of my head; I'm still not sure if
it is 'good' SGML, but SGML seems not to handle dynamic conditions
in a clean way:

1. Marked Sections

Clients by default can define the following types of general entities:


For example, a level 2 comforming client would imply the following


Then a document writer can use the following constructs:

<p>Blah blah blah

<![ &HTML.Level2; [
<!-- Whatever level 2 stuff in here -->
<![ &HTML.Level3; [
<!-- Whatever level 3 stuff in here -->

To provide alternatives for the same data.

If a client discovers a entity reference as the status-keyword and the
entity is not defined (like HTML.Level3 for level 2 or lower clients)
the replacement value 'IGNORE' is used.

2. Processing instructions

Processing instructions could be used to inform the client on different
levels of HTML being used in a document. For example:

<!-- Level 2 (and lower) markup here -->

<!-- Level 3 (and lower) markup here -->

<!-- Switch back -->

The client can send a warning to the user when higher level
instructions are specified that is not directly supported by the
client. The default level can be implied by the <!doctype declaration,
or if not defined, the highest level supported by the client.

Of course, there are inherent disadvantages to using processing
instructions, and this seems a little ugly to me.

3. New HTML elements

Add new elements to HTML like the following:


As you might guess, it is somthing similiar to a multipart/alternative
MIME message. Example of usage:

<part level=2>
<!-- level 2 markup -->
<part level=3>
<!-- level 3 markup -->

Clients will choose the markup that is contained in the highest level
part to render.

4. The CONCUR construct

The CONCUR feature of SGML can be used to handle alternate views. I'm
not to knowledgable/experienced on the CONCUR feature of SGML, so I'm
unsure if it can solve the problem. Plus, I think it would be awkward
for authors to use.

My $0.02,