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Re: ex as a unit

From: Stephen Deach <sdeach@Adobe.COM>
Date: Sat, 15 Apr 2000 10:54:33 -0700
Message-Id: <>
To: "Pawson, David" <DPawson@rnib.org.uk>, "'xsl-editors@w3.org'" <xsl-editors@w3.org>
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 analogour concept).
 2.) Most typefaces in widespread distribution have no "ex" height in the
font metric informations, 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 wdespread
typefaces and coposition 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 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 with CSS & WAI
representatives in Sophia-Antipolis, May 5, 1999; but 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 

This e-mail reflects the personal opinion of the author.
 -- Unless explicitly so stated in the text, it does not represent an
    official position of Adobe Systems, Inc.
 -- Unless explicitly so stated in the text, it does not represent an
    official opinion of the W3C XSL Working group.
  Stephen Deach                            |  Sr Computer Scientist
  408-536-6521 (office)                    |  Adobe Systems Inc.
  408-537-4214 (fax)                       |  Mail Stop W15-424
  sdeach@adobe.com (no ads)                |  345 Park Ave
                                           |  San Jose, CA 95110-2704
                                           |  USA
Received on Saturday, 15 April 2000 13:51:02 UTC

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