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Pixels vs Points as a unit for sizes

From: Charles McCathieNevile <charles@w3.org>
Date: Tue, 8 Jan 2002 01:28:43 -0500 (EST)
To: WAI IG <w3c-wai-ig@w3.org>
Message-ID: <Pine.LNX.4.30.0201080122190.11352-100000@tux.w3.org>
I think this is part of a Frequently asked Question...

Using px (pixels) as a unit of measure for font sizes isn't a great idea. It
takes no account of the size the user prefers to have their text so it can be
easily read. It also means your text is likely to bo very different sizes on
different kinds of system, so it doesn't provide control anyway.

Using pt (points, or 72nds of an inch) isn't too good either. Not only does
it ignore the size that the user would like to work with, but because of
strange bugs in some systems it isn't actually reliable in all browsers, and
you text may come out a lot smaller than expected.

There are a couple of cases where it seems reasonable to use these measures:

For images that do not scale well (e.g. gif, png or jpg formats) it is not a
bad idea to set sizes in terms of pixels.

For a narrow decorative border or horizontal line, it doesn't make that much
difference what the units are.

just a thought - my personal opinion.



Vadim Plessky recently wrote:

Now I realize that during 1.5 months that I am subscribed to this list, none
was asking "how using pixels instead of points affects site accessibility"
I am not visually impared, but have some preblems with accessing
microsoft.com site (and several other sites produced with genius "Microsoft
Can someone take care about me, please? :-)

What I found is that even if you use 'pt' font sizes - there is no warranty
that your system (Windows / PC or Macintosh) has correct dpi (dots per inch)
settings. And without correct dpi settings, there is no way how browser can
recalculate 'pt' (or 'cm', 'mm') to number of pixels...
I have read in some book /article that both Windows/PC and Macintosh are
broken in terms of reporting correct dpi settings. Has someone opposite info?

[ good news: recent versions of XFree86, used on Linux, can auto-detect dpi
on many video adpaters/display combination, so we finally have at least one
platform working ok]

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, 8 January 2002 01:28:42 UTC

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