Basically, I do something along the lines of:

• apply all filters
• do php2xml
• if display is xml, return xml
• do xml2html
• echo html

I might bypass the filtering system entirely in a next version of my theme, and do something along the lines of the following instead:

• $post = $post->apply_filters();
• … (as previous)

as suggested in the hackers list a couple of days ago. this would assume a limited number of use cases could be defined. I can name three:

• ‘main’ e.g. for use as a tile on a single page
• ‘abstract’ e.g. for use in a list as a search result
• ‘title’, e.g. for use in a list where only a title and subtitle are needed

there are possibly a few more. but not that many. in any event, this would significantly reduce the amount of plugin hooks, and do so for the better.

Perhaps this could be done by defining an [XML][] format for the text,
and then all filters would modify this? One could require that the
text is [XHTML][] at the beginning of the filter chain, and then the
filters should have access to a proper parser and only modify the text
through that. I’m not that much into this stuff, but I believe that
this is what the XSL Transformation language lets you do.

Of course, having all plugins modify the text using those style sheets
would make it a lot harder to write plugins. Perhaps using full
stylesheets should be optional — with the knowledge that the text is
(valid!) XHTML, the plugins will still be able to parse and change it
reliably without necessarily having to use an XML parser.

I guess my ideas goes somewhat along the lines of the changes
outlined by Michel Fortin on the mailinglist: draw a line
somewhere and say: “Here we deal with the raw input from the user,
and here we deal with valid XHTML.” If such a division was actually
made, then lots of things would be easier.