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Filter Levels
    When one size *does not* fit all

It makes little sense to constrain users to one set of HTML elements and
attributes and tell them that they are not allowed to mold this in
any fashion.  Many users demand to be able to custom-select which elements
and attributes they want.  This is fine: because HTML Purifier keeps close
track of what elements are safe to use, there is no way for them to
accidently allow an XSS-able tag.

However, combing through the HTML spec to make your own whitelist can
be a daunting task.  HTML Purifier ought to offer pre-canned filter levels
that amateur users can select based on what they think is their use-case.

Here are some fuzzy levels you could set:

1. Comments - Wordpress recommends a, abbr, acronym, b, blockquote, cite,
    code, em, i, strike, strong; however, you could get away with only a, em and
    p; also having blockquote and pre tags would be helpful.
2. BBCode - Emulate the usual tagset for forums: b, i, img, a, blockquote,
    pre, div, span and h[2-6] (the last three are for specially formatted
    posts, div and span require associated classes or inline styling enabled
    to be useful)
3. Pages - As permissive as possible without allowing XSS.  No protection
    against bad design sense, unfortunantely.  Suitable for wiki and page
    environments. (probably what we have now)
4. Lint - Accept everything in the spec, a Tidy wannabe. (This probably won't
    get implemented as it would require routines for things like <object>
    and friends to be implemented, which is a lot of work for not a lot of
    benefit)

One final note: when you start axing tags that are more commonly used, you
run the risk of accidentally destroying user data, especially if the data
is incoming from a WYSIWYG editor that hasn't been synced accordingly. This may
make forbidden element to text transformations desirable (for example, images).



== Element Risk Analysis ==

Although none of the currently supported elements presents a security
threat per-say, some can cause problems for page layouts or be
extremely complicated.

Legend:
    [danger level] - regular tags / uncommon tags ~ deprecated tags
    [danger level]* - rare tags

1 - blockquote, code, em, i, p, tt / strong, sub, sup
1* - abbr, acronym, bdo, cite, dfn, kbd, q, samp
2 - b, br, del, div, pre, span / ins, s, strike ~ u
3 - h2, h3, h4, h5, h6 ~ center
4 - h1, big ~ font
5 - a
7 - area, map

These are special use tags, they should be enabled on a blanket basis.

Lists - dd, dl, dt, li, ol, ul ~ menu, dir
Tables - caption, table, td, th, tr / col, colgroup, tbody, tfoot, thead

Forms - fieldset, form, input, lable, legend, optgroup, option, select, textarea
XSS - noscript, object, script ~ applet
Meta - base, basefont, body, head, html, link, meta, style, title
Frames - frame, frameset, iframe

And tag specific notes:

a   - general problems involving linkspam
b   - too much bold is bad, typographically speaking bold is discouraged
br  - often misused
center - CSS, usually no legit use
del - only useful in editing context
div - little meaning in certain contexts i.e. blog comment
h1  - usually no legit use, as header is already set by application
h*  - not needed in blog comments
hr  - usually not necessary in blog comments
img - could be extremely undesirable if linking to external pics (CSRF, goatse)
pre - could use formatting, only useful in code contexts
q   - very little support
s   - transform into span with styling or del?
small - technically presentational
span - depends on attribute allowances
sub, sup - specialized
u   - little legit use, prefer class with text-decoration

Based on the riskiness of the items, we may want to offer %HTML.DisableImages
attribute and put URI filtering higher up on the priority list.


== Attribute Risk Analysis ==

We actually have a suprisingly small assortment of allowed attributes (the
rest are deprecated in strict, and thus we opted not to allow them, even
though our output is XHTML Transitional by default.)

Required URI - img.alt, img.src, a.href
Medium risk - *.class, *.dir
High risk - img.height, img.width, *.id, *.style

Table - colgroup/col.span, td/th.rowspan, td/th.colspan
Uncommon - *.title, *.lang, *.xml:lang
Rare - td/th.abbr, table.summary, {table}.charoff
Rare URI - del.cite, ins.cite, blockquote.cite, q.cite, img.longdesc
Presentational - {table}.align, {table}.valign, table.frame, table.rules,
    table.border
Partially presentational - table.cellpadding, table.cellspacing,
    table.width, col.width, colgroup.width


== CSS Risk Analysis ==

Currently, there is no support for fine-grained "allowed CSS" specification,
mainly because I'm lazy, partially because no one has asked for it. However,
this will be added eventually.

There are certain CSS elements that are extremely useful inline, but then
as you get to more presentation oriented styling it may not always be
appropriate to inline them.

Useful - clear, float, border-collapse, caption-side

These CSS properties can break layouts if used improperly. We have excluded
any CSS properties that are not currently implemented (such as position).

Dangerous, can go outside container - float
Easy to abuse - font-size, font-family (font), width
Colored - background-color (background), border-color (border), color
    (see proposal-colors.html)
Dramatic - border, list-style-position (list-style), margin, padding,
    text-align, text-indent, text-transform, vertical-align, line-height

Dramatic elements substantially change the look of text in ways that should
probably have been reserved to other areas.
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