Re: CSS3 Text and UAX14

From: fantasai (
Date: Tue Feb 20 2007 - 07:10:43 CST

  • Next message: fantasai: "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:

    >> 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.


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