- Update filter-levels document to cover CSS and attributes

- Add colors proposal, for constraining allowed colors in  document
- Add strictness proposal, for attributes that are permitted by Transitional but not by HTML Purifier

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@442 48356398-32a2-884e-a903-53898d9a118a

docs/colors.txt

@@ -0,0 +1,23 @@
+
+Colors
+    Hammering some sense into those content-makers
+
+Your website probably has a color-scheme. Green on white, purple on yellow,
+whatever. When you give users the ability to style their content, you may
+want them to keep in line with your styling. If you're website is all
+about light colors, you don't want a user to come in and vandalize your
+page with a deep maroon.
+
+This is an extremely silly feature proposal, but I'm writing it down anyway.
+
+What if the user could constrain the colors specified in inline styles? You
+are only allowed to use these shades of dark green for text and these shades
+of light yellow for the background. At the very least, you could ensure
+that we did not have pale yellow on white text.
+
+Implementation issues:
+1. Requires the color attribute definition to know, currently, what the text
+and background colors are. This becomes difficult when classes are thrown
+into the mix.
+2. The user still has to define the permissible colors, how does one do
+something like that?

docs/filter-levels.txt

@@ -20,15 +20,32 @@ can further be customized using simpler configuration options.
 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, b and
-    i; also having p and pre tags would be helpful.
-2. Pages - As permissive as possible without allowing XSS.  No protection
+    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.
-3. Lint - Accept everything in the spec, a Tidy wannabe.
+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)
 
-I've also decomposed tags into risk levels.  An asterisk indicates that no one
-really uses that tag, tilde indicates it's deprecated.
+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 eidtor that hasn't been synced accordingly. This may
+make forbidden element to text transformations desirable (for example, images).
+
+
+
+== Element Risk Analysis ==
+
+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
@@ -38,30 +55,76 @@ really uses that tag, tilde indicates it's deprecated.
 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
+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
+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?
+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
+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 ==
+
+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
+Dramatic - border, list-style-position (list-style), margin, padding,
+    text-align, text-indent, text-transform, vertical-align, line-height
+
+Dramatic elements substnatially change the look of text in ways that should
+probably have been reserved to other areas.

docs/strictness.txt

@@ -0,0 +1,25 @@
+
+Is HTML Purifier Strict or Transitional?
+    A little bit of helpful guidance
+
+Despite the fact that HTML Purifier professes only to support transitional
+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. HTML Purifier does not currently support them, but
+it might behoove us to do so while our output is still 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.
+
+It's an icky question, and we'll have to deal with it as more and more 
+transforms get implemented.