Re: ex as a unit

[I have a bad keyboard -- typos corrected in this copy. I also made some
modifications to the wording of items 4 & 5. ---SDeach]

At this time, this is a personal opinion, and does not reflect a decision
of the XSL-WG. This is provided as input for that evaluation.


There are multiple problems with "ex" as a unit:
 1.) It is only meaningful for latin-based (and similar) typefaces. It is
not useful for Arabic, Indic, Ideographic or a number of other scripts. In
fact there is no way to determine the ex" height (or analogous measurement)
of many of these faces (since there is no analogous concept).
 2.) Most typefaces in widespread distribution have no "ex" height in the
font metric information, it must be derived by inspecting the character
shapes of selected letters (usually the lower-case x).
 3.) Type size is specified using the typebody height for all widespread
typefaces and composition systems, thus EM is a useful and well-defined
unit. If one wished to scale up the layout based on type-size, the EM is a
much better choice since it avoids disproportionate changes in line-spacing
(resulting in either disproportionately wide line-spacing or
overlapping-lines) and layout that occur if EX-Height is used. To fix this
disproportionate spacing, one must consult the EM-height anyway, so it is
better to use EM units in the first place.
 4.) Though ex-height has some value in determining the range of size
adjustment [especially the minimum size] for legibility of latin-related
typfaces, the font-size-adjust property seems an adequate mechanism for
doing this. Retaining "ex" as a general unit does not seem necessary.
 5.) This decision was made in a joint meeting of the XSL-WG with CSS & WAI
representatives in Sophia-Antipolis, May 5, 1999; but the change was not
reflected in the April 1999 draft. [Two votes were recorded in the
Wednesday minutes under discussion of Item 2:
  Proposal: That the XSL WG not implement exes; no objection.
  Proposal: That the XSL WG recommend that the CSS WG withdraw or
            deprecate the use of exes; no objection.
]




At 10:19 2000-04-04 +0100, Pawson, David wrote:
>I note that the ex has gone as a unit since 
>the January WD, its no longer listed in 
>the 27 Mar WD
>
>If the em is to be included, then surely the ex
>should also be available. My use case is for 
>scaling print sizes up for users of large print.
>
>Regards, DaveP 
>
>
>

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Received on Saturday, 15 April 2000 13:58:43 UTC