W3C home > Mailing lists > Public > www-style@w3.org > October 2010

Re: about the use of pixels as a unit for typography

From: Shelby Moore <shelby@coolpage.com>
Date: Tue, 12 Oct 2010 11:14:24 -0400
Message-ID: <1491a9f88b5be2e26473035ca2e98d4c.squirrel@sm.webmail.pair.com>
To: "Peter Moulder" <peter.moulder@monash.edu>
Cc: www-style@w3.org

You enumerate important issues, and that is very thought provoking. Thanks.

The em unit is useful for referential transparency (modularity), so the
class employing em can be applied where different font sizes are set.
Perhaps we should recommend not using em where the font size is the
browser's default.

We need a unit which is proportional to the viewer's comfortable
psychophysic viewing size at current resolution on that device. To relieve
us from deprecating pt, I would suggest that unit be equivalent
psychophysically to 1 pt on book printed pages, so we can just re-use pt
for this purpose.

Browsers should do their best to implement this psychophysic consistency,
employing the best science and OS APIs available. By making such a demand
in the specification, perhaps we can nudge the device and OS markets

By psychophysic consistency, I mean taking into account the best science
relating comfortable font reading size with respect to device screen size,
resolution, likely viewing distance, and any accessibility user
configuration. Leave the specifics of the psychophysic consistency
unspecified, for devices to optimize and compete on.

Note the designer may want to institute different designs depending on
factors relating to this psychophysic unit as measured in CSS pixels and
available resolution in CSS pixels. Does the designer need to do this with
JavaScript detect, or can we provide some way to select different designs
via CSS?

> On Tue, Oct 12, 2010 at 05:29:44PM +0800, David Singer wrote:
>> Don't forget that a CSS pixel might not be a single device pixel.
> This is important, but it doesn't help much unless device pixel size is
> near an
> integer fraction of a reference px: otherwise there's still too much
> difference
> between a reference px and an actual px, so still too much variation
> between
> different devices.
> When changes to units in CSS were last being discussed, I did wonder
> whether we
> should drop the "whole number of device pixels" recommendation from
> syndata.html, such that a px would equal a reference px.  The main reason
> I
> didn't in the end post this suggestion was concern that bitmapped images
> such
> as gifs (especially small ones) would still be with us for a long time.
> However, I do have some hope that images will come to be in a more
> resolution-independent form to accommodate pinch-based interfaces on
> handheld devices.
> Should there be a refpx unit (1/2688 of viewing distance) that could be
> used
> at CSS authors' discretion?
> In the shorter term, any recommendation of "use pt instead of px" would
> have to
> come with some caveats.  First of all the trivial caveat that I believe
> Gecko
> now treats pt as a fixed ratio of px (i.e. I believe recent builds treat
> 12pt
> as equivalent to 16px regardless of any knowledge it has of display pixel
> density).  If you instead had access to a real device pt (or mozmm) unit,
> then
> the problem would instead be that the physical pt size suitable for a
> desktop
> monitor at a "nominal arm's length" away won't be suitable for a handheld
> device that's less than half that distance away.
> And of course there's still the existing objections to pt/px sizes that
> the
> ideal size for something depends on the person reading it (e.g. how good
> their
> eyesight is) and what their display is (how big, how far away, whether it
> has
> good pixel density).
> Using the em unit would sound like a good choice, but I'm told that it too
> has
> problems if many of your audience have an unsuitable default font size and
> won't change it.
> Unfortunately there is no single ideal-for-all-purposes unit to recommend
> to
> designers.
> But certainly the variability of pixel densities is one factor for
> designers
> to consider.
> pjrm.
Received on Tuesday, 12 October 2010 15:14:52 UTC

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