Re: CSS3 Text and UAX14

On 2/20/2007 5:10 AM, fantasai wrote:
> Paul Nelson (ATC) wrote:
>> CSS is a higher level protocol. There is room for a higher level 
>> protocol to
>> override things, for example, by specifying the PRE.
> CSS is indeed a higher-level protocol. However, UAX14 sets explicit
> limits on what a higher-level protocol can do:
My position is that a higher level protocol is able to also offer 
non-compliant modes, such
as the unrestricted mode, or the emergency mode. Either one of these 
modes are by
design outside the limitations since the protocol would not "/purport to 
implement the Unicode Line Breaking Algorithm"/ for those modes.
>>> 1. Spaces are a non-tailorable line breaking class. The description 
>>> of its
>>> behavior also includes prescriptions on presentation that are not
>>> compatible with what CSS prescribes.
>> The only place where I see problems with the SP definition are in the 
>> PRE
>> situation where we are keeping the widths of all spaces explicitly. 
>> In this
>> case are we really tailoring the line breaking class of the character?
> AFAICT, there's only two ways of tailoring a class: changing its 
> membership
> (which is forbidden for SP), or changing the rules in 6.2. The statements
> governing the presentation of spaces are in 5.1...
I need to know specifically which behavior you think would need to change.
>>> 2. CSS has a line breaking mode that forbids all breaks. This needs 
>>> to override the non-tailorable behavior of the ZW (and SP?) classes.
>> In this case, CSS is simply saying that the line has no end, and 
>> therefore
>> there is no wrapping point. We are not overriding the behavior of the 
>> ZW and
>> SP classes.
> Hm, good point. :)
Yes, this was particularly elegant.
>>> 3. CSS3 Text introduces an 'unrestricted' line breaking mode. In 
>>> this mode,
>>> line breaking restrictions are ignored completely, (except for the CM
>>> class).
>> One way to look at the 'unrestricted' line breaking is that we are 
>> forcing
>> emergency line breaking to happen at the end of every line.
> Emergency line breaking is only allowed "if a legal line break cannot 
> be found".
> We could tailor everything tailorable into the AL class, but ZWSP is 
> still
> defined to always provide a legal line break. SP also can't be altered.
I think the better way to handle this is as a special (non-UAX14) mode. 
so makes it easier to give it precisely the specifications that are 
correct for CSS.

The reason that I have always resisted to put a formal description of 
the emergency
mode behavior into UAX#14 is that it's not clear to me whether a single
preferred method exists, or whether it would have to be tailorable itself.

I think emergency modes don't arise from the nature of the characters, so
the role of the UTC as owner of the character properties in standardizing
character behavior in this case is not so clear.

> ~fantasai

Received on Tuesday, 20 February 2007 22:42:57 UTC