htmlpurifier/docs/ref-loose-vs-strict.txt

Loose versus Strict
	[rename/deprecation pending]

The most common change between doctypes are between the two flavors of HTML 4.01 and
XHTML 1.0: Transitional (Loose) and Strict. Besides deprecated attributes and elements
(which are quite easy to identify), there are two content model changes that were
made:

BLOCKQUOTE changes from 'flow' to 'block'
    current behavior: inline inner contents should not be nuked, block-ify as necessary
ADDRESS from potpourri to Inline (removes p tags)
    current behavior: block tags silently dropped
    ideal behavior: replace block elements with something like <br>. (not high priority,
        somewhat difficult to implement)

We're missing strict support for U, S, STRIKE: this needs to be fixed soon (and
is quite simple to fix).