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RE: ems versus pixels

From: Charles McCathieNevile <charles@w3.org>
Date: Tue, 1 Jan 2002 17:47:07 -0500 (EST)
To: Rowan Smith <rowan@absolutely.co.nz>
cc: <w3c-wai-gl@w3.org>
Message-ID: <Pine.LNX.4.30.0201011742100.26154-100000@tux.w3.org>
Hi Rowan,

well, users who are themselves relying on speech output often get that from
software that reads the content as presented on the screen, rather than using
software that provides a true audio interface. So anything affecting visual
presentation will affect the presentation for many screen reader users.

But the major problems will in fact be for visual users - if the screen is
magnified, 4 or 5 times, or if the font size is set in the region of 30 or 40
pt, or if the window size is changed between 320x240 and 1200x800 is there
any problem? (In X window systems setting the screen size at 320x240 or a bit
smaller is an easy way of getting effective screen magnification, when it is
rendered on a large screen.

Cheers

Charles

On Wed, 2 Jan 2002, Rowan Smith wrote:

  Hi Charles

  I'm using layers as a means of positioning blocks that make up elements of a
  page (which contain text), rather than as a means of controlling the text
  directly. Changes to text size etc should still fit within the blocks. The
  page in question degrades logically in the absence of styles, so I guess my
  question was about maintaining the visual integrity (ie layout) of an
  accessible page.

  Given that my (intended) use of px was to preserve visual appearance, I was
  wondering if that got in the way of users who don't rely on the visual but I
  can't think why it should.

  Cheers
  Rowan
  -----Original Message-----
  From: Charles McCathieNevile [mailto:charles@w3.org]
  Sent: Wednesday, 2 January 2002 2:46 a.m.
  To: Rowan Smith
  Cc: w3c-wai-gl@w3.org
  Subject: Re: ems versus pixels


  Well, the accessibility downside depends on what you are soing with your
  layers. If you are going to have text positioning determined by them, and
  people changing text sizes or window sizes to suit their needs is going to
  make a big mess, then the accessibility downside is that the design (not
  just
  using pixels, which is really a fairly small part) isn't going to work. In
  this case I would suggest changing the design to start with, to one where it
  does work to specify sizes in em units.

  If you are trying to put a pretty border around the edge of a page, or place
  a picture that doesn't scale properly anyway, then there is probably no real
  problem that could be solved by switching to em units.

  As I undertstand it the techniques document, where you have obviously
  looked,
  is a FAQ, and raising further issues on it means that there is a need for
  some more material there.

  Cheers

  Charles

  On Tue, 1 Jan 2002, Rowan Smith wrote:

    Hi there

    WAI Priority 3 guidelines (WCAG Checkpoint 3.4) recommend using relative
    units rather than absolute. The Techniques document suggests that using
  ems
    rather than pixels as a unit is a way of doing this even when using
  absolute
    positioning.

    OK, I can understand that for text specifications like font sizes and line
    heights, but does it apply to positioning layers on a page using CSS? Is
    there an accessibility downside of positioning layers (div tags) by using
    px?

    Thanks

    Rowan


  --
  Charles McCathieNevile    http://www.w3.org/People/Charles  phone: +61 409
  134 136
  W3C Web Accessibility Initiative     http://www.w3.org/WAI    fax: +1 617
  258 5999
  Location: 21 Mitchell street FOOTSCRAY Vic 3011, Australia
  (or W3C INRIA, Route des Lucioles, BP 93, 06902 Sophia Antipolis Cedex,
  France)



-- 
Charles McCathieNevile    http://www.w3.org/People/Charles  phone: +61 409 134 136
W3C Web Accessibility Initiative     http://www.w3.org/WAI    fax: +1 617 258 5999
Location: 21 Mitchell street FOOTSCRAY Vic 3011, Australia
(or W3C INRIA, Route des Lucioles, BP 93, 06902 Sophia Antipolis Cedex, France)
Received on Tuesday, 1 January 2002 17:47:10 GMT

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