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 ==

BLOCKQUOTE changes from 'flow' to 'block'
    behavior: inline inner contents should not be nuked, paragraph as necessary
U, S, STRIKE cut
    behavior: replace with appropriate inline span + CSS
ADDRESS from potpourri to Inline (removes p tags)
    behavior: p tags silently dropped or replaced with something (<br>)

== Things we can loosen up ==

Tags DIR, MENU, CENTER, ISINDEX, FONT, BASEFONT? allowed in loose
Attributes allowed in loose:
    div,p,h#.align
    ul,ol.type
    ul,ol,dl.compact
    ol.start
    li.type,value
    hr.align,noshade,size,width
    pre.width
    img.name?,align,border,hspace,vspace
    table.align,bgcolor
    caption.align
    tr.bgcolor
    th,td.nowrap,bgcolor,width,height

== Periphery issues ==

A tag's attribute 'target' (for selecting frames) cut
    behavior: shouldn't be needed, use loose doctype if needed
OL/LI tag's attribute 'start' (for renumbering lists) cut
    behavior: no substitute, just delete
Attribute 'name' deprecated in favor of 'id'
    behavior: create proper AttrTransform
PRE tag allows SUB/SUP? (strict dtd comment vs syntax, loose disallows)