Re: CSS3 Text and UAX14

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:

   http://unicode.org/reports/tr14/#ConfProtocols

>> 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...

>> 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. :)

>> 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.

~fantasai