htmlpurifier/docs/enduser-id.txt

IDs
    What they are, why you should(n't) wear them, and how to deal with it

Prior to HTML Purifier 1.2.0, this library blithely accepted user input that
looked like this:

    <a id="fragment">Anchor</a>

...presenting an attractive vector for those that would destroy standards
compliance: simply set the ID to one that is already used elsewhere in the
document and voila: validation breaks.  There was a half-hearted attempt to
prevent this by allowing users to blacklist IDs, but I suspect that no one
really bothered, and thus, with the release of 1.2.0, IDs are now *removed*
by default.

IDs, however, are quite useful functionality to have, so if users start
complaining about broken anchors you'll probably want to turn them back on
with %HTML.EnableAttrID. But before you go mucking around with the config
object, it's probably worth to take some precautions to keep your page
validating. Why?

1. Standards-compliant pages are good
2. Duplicated IDs interfere with anchors.  If there are two id="foobar"s in a
   document, which spot does a browser presented with the fragment #foobar go
   to? Most browsers opt for the first appearing ID, making it impossible
   to references the second section. Similarly, duplicated IDs can hijack
   client-side scripting that relies on the IDs of elements.

You have (currently) four ways of dealing with the problem.



Road #1: Blacklisting IDs 
    Good for pages with single content source and stable templates

Keeping in terms with the KISS (Keep It Simple, Stupid) principle, let us
deal with the most obvious solution: preventing users from using any IDs that
appear elsewhere on the document.  The method is simple:

    $config->set('HTML', 'EnableAttrID', true);
    $config->set('Attr', 'IDBlacklist' array(
        'list', 'of', 'attributes', 'that', 'are', 'forbidden'
    ));

That being said, there are some notable drawbacks.  First of all, you have to
know precisely which IDs are being used by the HTML surrounding the user code.
This is easier said than done: quite often the page designer and the system
coder work separately, so the designer has to constantly be talking with the
coder whenever he decides to add a new anchor.  Miss one and you open yourself
to possible standards-compliance issues.

Furthermore, this position becomes untenable when a single web page must hold
multiple portions of user-submitted content.  Since there's obviously no way
to find out before-hand what IDs users will use, the blacklist is helpless.
And even since HTML Purifier validates each segment seperately, perhaps doing
so at different times, it would be extremely difficult to dynamically update
the blacklist inbetween runs.

Finally, simply destroying the ID is extremely un-userfriendly behavior: after
all, they might have simply specified a duplicate ID by accident.

Thus, we get to our second method.



Road #2: Namespacing IDs
    Lazy developer's way, but needs user education

This method, too, is quite simple: add a prefix to all user IDs. With this
code:

    $config->set('HTML', 'EnableAttrID', true);
    $config->set('Attr', 'IDPrefix', 'user_');

...this:

    <a id="foobar">Anchor!</a>

...turns into:

    <a id="user_foobar">Anchor!</a>

As long as you don't have any IDs that start with user_, collisions are
guaranteed not to happen.  The drawback is obvious: if a user submits
id="foobar", they probably expect to be able to reference their page with
#foobar. You'll have to tell them, "No, that doesn't work, you have to add
user_ to the beginning."

And yes, things get hairier.  Even with a nice prefix, we still have done
nothing about multiple HTML Purifier outputs on one page.  Thus, we have
a second configuration value to piggy-back off of: %Attr.IDPrefixLocal:

    $config->set('Attr', 'IDPrefixLocal', 'comment' . $id . '_');

This new attributes does nothing but append on to regular IDPrefix, but is
special in that it is volatile: it's value is determined at run-time and
cannot possibly be cordoned into, say, a .ini config file.  As for what to
put into the directive, is up to you, but I would recommend the ID number
the text has been assigned in the database.  Whatever you pick, however, it
has to be unique and stable for the text you are validating.  Note, however,
that we require that %Attr.IDPrefix be set before you use this directive.

And also remember: the user has to know what this prefix is too!



Path #3: Abstinence

You may not want to bother. That's okay too, just don't enable IDs.

Personally, I would take this road whenever user-submitted content would be
possibly be shown together on one page.  Why a blog comment would need to use
anchors is beyond me.



Path #4: Denial

To revert back to pre-1.2.0 behavior, simply:

    $config->set('HTML', 'EnableAttrID', true);

Don't come crying to me when your page mysteriously stops validating, though.