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   Re: [xml-dev] Processing instruction guidelines

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/ Jeni Tennison <jeni@jenitennison.com> was heard to say:
| Are there any other good uses for PIs?

Absolutely. I am totally exasperated by the folks that want to remove
PIs from XML. "Here's your Swiss Army Knife, norm, oh, but we broke
off the small blade (the internal subset) and we've removed the
tweezers (PIs), because you don't really need those. And for good
measure we welded the corkscrew open (thou shalt always put elements
in a namespace). Is there anything else we can do to help you?"

| Does anyone have examples of
| *inappropriate* uses? I have the impression that some people think
| that any use of PIs is inappropriate, but I'm not sure what the
| argument is?

Inappropriate? Hmm. I'd say that it was inappropriate to put proper
information content in there:

  <p>This is a paragraph</p>
  <?p This is a special paragraph that is really important but I jammed it in a PI?>

That'd be bad.

| Also, are there any guidelines about naming the targets of PIs? The
| ones that I've seen tend to use a naming scheme "prefix-localPart"
| where the prefix is used to indicate the authority or application
| involved and the local part is used to specify the name of the
| instruction itself. Is that fairly standard?

More-or-less. Maybe PI targets should have been allowed to be QNames,
I don't know. Clearly they have to be named with the same
considerations as they are a flat space. I've used "dbhtml" and "dbfo"
in the DocBook stylesheets which may be a bit too broad.

What can you do with the PIs in the DocBook styleheets?

- - Provide hints for line numbering of verbatim environments
- - Provide "table-summary" text for HTML tables. This was a bad use
  and is now deprecated since DocBook added a proper element to allow this,
  but the PI was a workable solution before the schema was revised.
- - Provide hints for the cellspacing and cellpadding of rendered HTML tables
- - Control the style of function synopses (K&R or ANSI)
- - Toggle ToCs for Q&A sets
- - Provide some suggested widths in description list environments
- - Specify the filename to use for "chunked" documents
- - Control presentation of some lists as blocks or tables
- - Specify rotation for table cells
- - Specify background color for table cells

| Despite the fact that the XML Rec. suggests using notations to specify
| the legal names of PIs, I haven't actually seen that in use. Anyone
| think that it's good practice?

Notations, alas, are a good idea that never really took off. And since
the WXS idea of notations and the DTD idea are pretty different,
they're probably dead.

| What about PI data? Most PIs that I've seen use pseudo-attributes, I
| guess for extensibility. Is that standard practice?

I think so. Certainly I've seen other styles, but as a stylesheet
author, it's very convenient. The alternative is either some other
tokenizing strategy or a different PI target name for each possible

                                        Be seeing you,

- -- 
Norman.Walsh@Sun.COM    | As we grow older we grow both more foolish
XML Standards Architect | and wiser at the same time.--La Rochefoucauld
Web Tech. and Standards |
Sun Microsystems, Inc.  | 
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