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Re: Sizing

From: Bert Bos <bert@w3.org>
Date: Mon, 18 Sep 2006 20:25:36 +0200
To: www-style@w3.org
Message-Id: <200609182025.36973.bert@w3.org>

On Sunday 17 September 2006 13:07, Axel Dahmen wrote:
> I see a lack in the CSS specification regarding size which I believe
> should be discussed for CSS3:
> <- a ->
> Absolute sizes of generated content are defined as the generated
> content's size itself whereas relative sizes are interpreted as being
> relative to the containing block's calculated size. With the current
> specification it's not possible to scale images in DHTML by applying
> CSS attributes (given you don't/can't use DOM properties to retrieve
> the original size of generated content).
> A new rule should be introduced for generated content to determine
> relative sizes as being relative to the generated content's intrinsic
> size.

There are two proposals under consideration by the CSS working group:

1) 'image-resolution'

This applies to replaced elements that have a resolution, i.e., to 
raster images. (It would thus also apply to generated content, if it 
happens to be a raster image.) The syntax isn't certain yet, but here 
is an example:

'Image-resolution: normal' (the default) means to use 1 image pixel = 

'Image-resolution: auto' means to use the intrinsic resolution as given 
by meta-data inside the image file, or use 'normal' if there isn't any.

'Image-resolution: <number>' means to display the image at <number> dpi.

'Image-resolution: <length>' means to use that length for the size of 
image pixels. (In other words: 'normal' means the same as '1px' and 
'200' means the same as '0.005in'.)

You can also give two numbers, in case the pixels aren't square.

It's an open question how this applies to background images.

It's also an open question if 'auto' needs a user-specified fallback: 
'image-resolution: auto' falls back to 'normal', while 
'image-resolution: auto, 300' would fall back to 300 dpi.

There is support for this property in the WG and I expect it to be 
further developed. The typical use case is actually not to set a 
specific resolution, but to express trust in the intrinsic resolution 
of certain images ('image-resolution: auto'), because browsers by 
default don't trust it (that's why 'image-resolution: normal' is the 

2) 'width: <number>' (and 'height: number')

This applies to block-level elements and replaced elements.

If the element has an intrinsic width, use <number> times the intrinsic 
width as its width. If it doesn't, the effect is the same as 'width: 

This hasn't been discussed much yet. There are obviously 
interdependencies with 'image-resolution' and with 'crop'.

An author presumably knows both the size and the resolution of his 
(raster) images, so he could use 'width' instead of 'image-resolution', 
but setting 'image-resolution: auto' once for the whole document may be 
easier, especially if you want to write the style sheet before creating 
the images.

> <- b ->
> There is no way to scale a document, e.g. for printing.
> What is rendered on screen always varies depending on the screen
> resolution. Units like "pt" are never displayed in their correct
> sizes. Similar for printing: "px" unit rules are never printed in the
> printer's resolution.

"Never" isn't true. My browser uses the dpi of the display as reported 
by the operating system: 98 dpi in my case (while measuring with a 
ruler shows that it is actually 99), so 'pt' displays pretty much 

I also tried printing with various browsers and other formatters. They 
all printed 'pt' quite well. I found one that was 3% off, but others 
that were 0% or 0.5% off according to my ruler.

And I tried printing 'px' (on recent office-class laserprinters). I 
found one browser that used about 1/72 inch, but the other products 
used values between 1/96 and 1/99 inch. That's a bit too big a 'px' for 
my taste, but it is acceptable. Laserprinters of this type don't round 
values to whole dots (they can effectively print details at 1200 dpi or 
so by varying the size of the dots), so there is no need that a px is a 
multiple of the nominal resolution. Lines won't look any sharper.

  Bert Bos                                ( W 3 C ) http://www.w3.org/
  http://www.w3.org/people/bos                               W3C/ERCIM
  bert@w3.org                             2004 Rt des Lucioles / BP 93
  +33 (0)4 92 38 76 92            06902 Sophia Antipolis Cedex, France
Received on Monday, 18 September 2006 18:25:53 UTC

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