== The _id_ attribute considered dangerous

I've been doing various overhauls of DWiki recently (a faster and
better DWikiText to HTML renderer written by Daniel Martin (and hacked
by me, so blame me for any problems) went in tonight, for example).
As part of this, I've been looking at the HTML DWiki generates,
running it past HTML Tidy and the [[W3C validator
http://validator.w3.org/]] and so on. It's been a learning
experience.

One of the things I've learned is that almost everywhere DWiki code
and templates were giving _<div>_s an _id="..."_ attribute, it was the
wrong thing to do. In something like DWiki, _id_ elements are
*dangerous*.

The problem for a dynamic, template driven site is that every
(hard-coded) _id_ is a promise that that particular element will only
ever appear ~~once~~ in a page. Any page. *Every* page. Better not
forgetfully reuse an _id_-containing template twice in some page, even
if it's the structurally right thing to do.

(For instance, the <div> around all of the comments on a page used to
have an _id_. That seemed harmless until I thought about a) possibly
showing comments by default on pages and then b) the blog view, which
would show multiple pages in their normal rendering glommed into one
bigger page. Kaboom!)

In DWiki's case, using _id_ wasn't getting me anything I couldn't get
with _class_; CSS styling can use either. This made my use of _id_
more or less gratuitous, which just made my mistakes with it all the
more wince-inducing. (The height of hubris was having a renderer, not
even a template, put an _id_ in its output.)

(I'm sure that advanced CSS tricks care about the difference, but
DWiki's use of CSS is pretty simple.)

After a bunch of revisions, I now have _id_ attributes in only five
places, and at least two of them (the page header and the syndication
feed information) are probably still mistakes. (They persist because
I'd have to revise the CSS too, and I've already made enough big
wrenching changes today.)

PS: changing the DWikiText renderer caused some fiddly little changes
in the HTML of every entry in CSpace's syndication feeds. If your feed
reader exploded as a result of this, you may want to get a better
one. (With a very few minor exceptions, there should be no visual
differences.)