htmlpurifier/docs/ref-strictness.txt

Is HTML Purifier Strict or Transitional?
    [rename/deprecation pending]

Despite the fact that HTML Purifier professes to support both transitional and
strict HTML, it rejects a lot of attributes and elements that are actually, indeed,
valid. You can investigate progress.html to find out precisely what we
are doing to these *deprecated* attributes.

However, users have found that Strict HTML imposes some quite unreasonable
restrictions on certain things. The start and value attributes in ol and
li (respectively) perhaps are the most contested. There's is currently no
widely supported browser method short of JavaScript that can replace these
two deprecated elements. It behooves us to allow these deprecated
attributes when the output is transitional.

Fortunantely, that's the only real bugger case. The others have near-perfect
CSS equivalents, and were presentational anyway. However, the other question
pops up: should we always convert these to the CSS forms when 1. the spec
allows them anyway and 2. older browsers support them better? After all, the
whole point about CSS is to seperate styling from content, so inline styling
doesn't solve that problem.

[new material]

HTML Purifier 1.7 creates a new organizational system for deprecated attribute/
element transformations. They will be unified under the title of "Tidy", which
is what they are: cleaning up after deprecated user markup into standards-compliant
versions. There will also be a change in the default behavior (athough, to the
end user not inspecting the HTML, there will be no change: in fact, it may
work even better).

Consult the Advanced API for more details.