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Re: [OT] Re: font size

From: Reagan D. Lynch <rdlynch@overland.net>
Date: Wed, 7 Apr 2004 21:22:26 -0500 (CDT)
Message-ID: <1155.>
To: <charles@sidar.org>
Cc: <w3c-wai-ig@w3.org>

Hi Chaals,

Thank you very much.

Yes, I was looking at this as an accessibility point.  I
design a site for a blindness group and up until now I've
been using pt.

From your explanation it sounds like my best bet is to use %.


Reagan D. Lynch

Charles McCathieNevile said:
> Hi Reagan,
> you might be better asking this question on a CSS list
> (there are  several around) although if you're
> particularly interested in the  accessibility aspects
> this is an appropriate place to ask. Anyway, here  is a
> rough guide...
> px, pt, cm, in, %, em and ex are different units for
> sizes.
> pt is points - there are 72 of them in an inch, and they
> don't change  size.
> cm and in are centimetres and inches - depending on
> whether you grew up  in the rest of the world in the
> last few decades, or grew up as a  british subject or an
> American, one or the other will be familiar. They  don't
> change size either.
> px is pixels - there are generally between 70 and 100 of
> them in an  inch. It really depends on the settings you
> have for your monitor  display, although there are
> several assorted ways of varying it. Some  browsers will
> let you change the size of a pixel by zooming in and
> out,  some don't, or make it extra hard. Generally you
> can't change the size  of a pixel by setting up a user
> style sheet.
> % is percentage. What happens when you set a font-size
> in % is that it  calculates based on the base font size.
> For example, if my default size  is 12pt (as is the case
> for many browsers) and you set font-size: 80%  my
> browser will try to render it as 9.6 pt (If you happen
> to be in  Australia, note that this is too small for a
> legal contract. People may  be able to ignore what ever
> they decide that is presented thus. More to  the point,
> many people everywhere find it too small to read
> comfortably).
> This effect accumulates. If I set the size of a list
> item to be 80%,  and the size of my body content to be
> 80%, then when the browser
> computes the size for the list item it will be 80% of
> 80% (because the  list item is  in the body and the body
> is already 80%) or 64%. (There  are easy ways around
> this - you just have to specify what happens when  you
> have combinations).
> em is similar - it is a unit based on a particular
> measure of the  character size. So 1em is equivalent to
> 100% (or whatever size things  were already), 0.8 em is
> equivalent to 80% and 2em is equivalent to  200%. (Sort
> of. The way it works is a bit more complex, and you get
> the  effects where mixing % and em is useful. See the
> earlier messages in  this thread for more links to
> information like this).
> ex is similar to em, although measured on a different
> part of the  character. It seems to be rarely used today
> on the Web.
> The point about % and em (and ex) is that they are based
> on the user's  font size - one of the things that (in
> theory) is very easy to change,  so setting your sizes
> in terms of them allows for easily flowing
> layouts when users change the size of their font. If you
> set sizes in  terms of pt, cm, mm, or in then in theory
> it is quite hard to change  them as a user (in practice
> this isn't always the case - there are  different ways
> of zooming things, but the results are not quite so
> predictable as an author).
> There is a lot of discussion over how easy or hard it is
> to change the  size of pixels, and whether these should
> be considered "relative units"  - a technical term used
> in a particular WCAG checkpoint whose goal is  to make
> it easy for the user to change the size of
> presentational  aspects of their browser (window size,
> font size) and have the content  come out looking
> reasonable. As you can see earlier in this thread...
> Hope that is helpful as a rough introduction. Please do
> follow up with  the links and information that is more
> detailed if you want to
> understand the intricacies of this area.
> cheers
> Chaals
> On 7 Apr 2004, at 14:18, Reagan D. Lynch wrote:
>> Hello,
>> I have a question about font size and css.
>> I understand what font-size: 12px would mean, but what
>> would fon-size: 12pt do?
>> Also, what is the difference between % and em?
>> And why would I be better off using font-size: 100%?
>> Thanks,
>> Reagan D. Lynch
> --
> Charles McCathieNevile
> Fundación Sidar
> charles@sidar.org
> http://www.sidar.org

Received on Wednesday, 7 April 2004 22:23:00 UTC

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