W3C home > Mailing lists > Public > www-style@w3.org > March 2011

Re: box tree terminology

From: Peter Moulder <peter.moulder@monash.edu>
Date: Wed, 30 Mar 2011 18:03:24 +1100
To: www-style@w3.org
Message-id: <20110330070324.GA26142@bowman.infotech.monash.edu.au>
On Tue, Mar 29, 2011 at 07:44:19PM -0700, fantasai wrote:
> We have three concepts in CSS that are used sometimes-interchangeably in CSS2.1.
> They need distinct terminology, but I haven't been able to come up with a good
> set. They are:
>   element
>     Either a source document element or a pseudo-element.
>   rendering tree thing
>     This is often referred to as a "box". Except for display: none elements,
>     which have none, and display: list-item elements, which have two, there
>     is only one "thing" per source document element.

Regarding how we split things up:

My understanding is that the tree that counters want to be defined by
should exclude display:none source elements (and probably those
descendants of display:table-column and display:table-column-group that
CSS 2.1 section 17.2.1 says should be treated as if they had
display:none; and possibly even display:table-column[-group] elements
themselves for some interpretation of "rendered exactly as if they had
display:none"); and should be affected by :before and :after, but
should probably not be affected by any :first-line or :first-letter
selectors.  I believe counters aren't affected by 'float' and
'position' properties.  (Note I haven't tested UA behaviour for most of
what's in this paragraph, and I forget what CSS3 says on the matter.)

Text defining selectors (when discussing what selectors/rulesets apply
to, as distinct from for purposes of :first-child etc.) wants a term
that includes source elements and all pseudo-elements, but without
regard for display values, and without including anonymous
elements/boxes.  One possibility here would be to use a term like
"subject" or "target" rather than an element-like term.

conform.html in CSS2.1 defines "element" to mean "source document
element".  A number of specifications defer to CSS2.1 for the
definitions in conform.html, so we ought to be careful in how we use
the unqualified word "element".

Text defining run-in (for css3-box) would prefer to define in terms of
a tree that's not affected by :first-line or :first-letter.  This is
similar to what [I believe] the definition of counters wants, except
that the run-in rules would want to use an "in-flow"-like qualifier.

Is "gen-element [tree]" any good as a term for [a node in] the
conceptual tree that includes the effects of :before and :after and
display:none-like things and excludes the effects of :first-line and
:first-letter and the splitting that results from bidi & line breaking?

There are probably no absolute requirements for which effects are
included in the tree referred to by which term: regardless of the
choice (short of having a dozen slightly different terms), there will
still be some need in the text for each effect, and each bit of text
using phrases like "sibling", "parent", "child" etc., to be clear about
how it's affected by other effects.  (Where "effects" here means things
that change parent/sibling relations: which in CSS 2.1 would include
anonymous block box stuff, table-related manipulations, and list
markers (especially inside markers), and arguably pagination; while
e.g. css3-content has some other things.)

Received on Wednesday, 30 March 2011 07:03:56 UTC

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