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

Re: Making pt a non-physical unit

From: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Date: Tue, 5 Jan 2010 18:22:44 +0100
Message-ID: <cb7bb73a1001050922i45d8aa16r85f4cc8c3dc4cf35@mail.gmail.com>
To: robert@ocallahan.org
Cc: www-style <www-style@w3.org>
On Tue, Jan 5, 2010 at 12:05 PM, Robert O'Callahan <robert@ocallahan.org> wrote:
> Various Web sites specify font sizes for some content in pt [1]. They look
> fine in most browsers, but a length in pt that looked the same as a px
> length when DPI was 96 will be 50% longer than that px length when the DPI
> is 144. Consequently, if a browser strictly adheres to the spec and treats
> 1pt as 1/72in, these sites look bad, broken, or unusable. The problem is
> especially acute for mobile devices, because their screens are often both
> high-density and small.
> My understanding from a conversation I had on #webkit is that Webkit avoids
> the problem by treating 1pt as 4/3px regardless of the display DPI. I think
> we probably need to do this in Gecko for Web compatibility reasons, and so
> for the sake of honesty in Web specifications, I propose that the definition
> of pt in CSS be altered accordingly.
> (At this time, I don't think we need to give up on physical units entirely;
> all the problematic sites I'm aware of are misusing pt, but not mm or in. mm
> remains useful for specifying the dimensions of touch-based interfaces.)
> [1] E.g. the search <select> on http://amazon.com; the title at
> http://blogs.mercurynews.com/kawakami; pretty much all the content at
> http://thenokiablog.com. You can see the results for yourself in Firefox by
> opening about:config and setting layout.css.dpi to 180 or so.

Honestly, I don't like this idea. Not only it is wrong in principle,
but it also breaks the layouts of anybody using pt correctly as a
physical unit; it also breaks uses of CSS beyond screen rendering,
unless you only plan to do the pt = 4/3px for screen rendering only.

Rather, aside from pressuring the websites of broken pages to fix
their stylesheets, I would rather see some way for UA to fix the
broken pages by either forcing a specific dpi or overriding the pt
value; but this would have to be done for specific pages, for example
the way Opera does this via browser js to fix sites that fail to work
in it.

Giuseppe "Oblomov" Bilotta
Received on Tuesday, 5 January 2010 17:23:37 UTC

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