Loose versus Strict
    Changes from one doctype to another

There are changes.  Wow, how insightful.  Not everything changed is relevant
to HTML Purifier, though, so let's take a look:

== Major incompatibilities ==

[done] BLOCKQUOTE changes from 'flow' to 'block'
    current behavior: inline inner contents should not be nuked, block-ify as necessary
[partially-done] U, S, STRIKE cut
    current behavior: removed completely
    projected behavior: replace with appropriate inline span + CSS
[done] ADDRESS from potpourri to Inline (removes p tags)
    current behavior: block tags silently dropped
    ideal behavior: replace tags with something like <br>. (not high priority)

== Things we can loosen up ==

Tags DIR, MENU, CENTER, ISINDEX, FONT, BASEFONT? allowed in loose
    current behavior: transform to strict-valid forms
Attributes allowed in loose (see attribute transforms in 'dev-progress.html')
    current behavior: projected to transform into strict-valid forms

== Periphery issues ==

A tag's attribute 'target' (for selecting frames) cut
    current behavior: not allowed at all
    projected behavior: use loose doctype if needed, needs valid values
[done] OL/LI tag's attribute 'start'/'value' (for renumbering lists) cut
    current behavior: no substitute, just delete when in strict, allow in loose
Attribute 'name' deprecated in favor of 'id'
    current behavior: dropped silently
    projected behavior: create proper AttrTransform (currently not allowed at all)
[done] PRE tag allows SUB/SUP? (strict dtd comment vs syntax, loose disallows)
    current behavior: disallow as usual