W3C home > Mailing lists > Public > www-style@w3.org > July 2000

RE: px vs. pt

From: Rowland Shaw <Rowland.Shaw@seagatesoftware.com>
Date: Thu, 27 Jul 2000 10:49:20 +0100
Message-ID: <81E198609B9DD311BE0A00508B5E2984440E3E@ipsent02.camelot.seagatesoftware.com>
To: "'webmaster@richinstyle.com'" <webmaster@richinstyle.com>
Cc: "'www-style@w3.org'" <www-style@w3.org>
> Rowland Shaw wrote:
> > What are people's views on preference?? I've become embroiled in a
couple of
> > debates recently where people have been defending the good old "px"
> > which I think could be potentially dangerous (well, from the point of
> > of supporting people with large screens and the disabled)
> The archives of the comp.infosystems.www.authoring.stylesheets contain
> extensive debates on this subject.

> > For reference, the units are defined as being:
> > 1pt == 1/72 inch (device independent for physical size)
> > 1px == 1 pixel   (device dependent for physical size)
> (No - see the belowcited URL) 

I think you'll find that's how they're defined in the CSS 1 spec as:
P  { font-size: 12px }    /* pixels, relative to canvas */
H1 { margin: 0.5in }      /* inches, 1in = 2.54cm */
H2 { line-height: 3cm }   /* centimetres */
H3 { word-spacing: 4mm }  /* millimetres */
H4 { font-size: 12pt }    /* points, 1pt = 1/72 in */
H4 { font-size: 1pc }     /* picas, 1pc = 12pt */

> > My personal preference would be to suggest using pt. (or pica or cm etc)
> I disagree strongly - see
> http://richinstyle.com/masterclass/lengths.html

Pretty, but it is flawed by the fact you're example was/is using large fonts
on a resolution intended for small fonts...

The size of a pixel is only *suggested* so cannot be guaranteed:
"Pixel units...are relative to the resolution of the canvas, i.e. most often
a computer display. If the pixel density of the output device is very
different from that of a typical computer display, the UA should rescale
pixel values. The suggested reference pixel is the visual angle of one pixel
on a device with a pixel density of 90dpi and a distance from the reader of
an arm's length. For a nominal arm's length of 28 inches, the visual angle
is about 0.0227 degrees"

> (Unfortunately most WYSIWYG tools do to - products such as Dreamweaver
> and all Microsoft products use points).

They transfer to print nicely like that...
IMHO, physical sizes should be used for root elements, and relative sizes
(in em or ex) for all the others...

> > On a Windows machine with "small fonts" a screen resoultion of 72dpi is
> > assumed, with "large fonts" it uses 96dpi - as for other platforms, I
> > say...

> Screenshots are at http://richinstyle.com/mac.png and
> http://richinstyle.com/windows.png
Received on Thursday, 27 July 2000 05:50:04 UTC

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