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Re: background-clip

From: Brad Kemper <brad.kemper@gmail.com>
Date: Tue, 8 Sep 2009 09:06:31 -0700
Cc: www-style list <www-style@w3.org>
Message-Id: <42D81E05-8539-4A0B-AE85-36B710D6C729@gmail.com>
To: L. David Baron <dbaron@dbaron.org>

On Sep 8, 2009, at 8:26 AM, L. David Baron wrote:

>> However, “Layering multiple background images” doesn't seem to  
>> mention
>> 'background-clip'. I think it should say something like the  
>> following:
>>
>> 	Regardless of how many background images are used, the box has  
>> only one
>> background painting area, as defined by the ''background-clip'  
>> property.
>> The background color, if present, is painted in this area, below  
>> all of
>> the other layers.
>
> I'm not convinced that there's anything inconsistent about the
> current spec; "sized, positioned, and tiled" could include the
> effects of background-clip.  And the current spec is certainly clear
> that background-clip takes multiple values, since it says:
>  Value: [border-box | padding-box ] [ , [border-box | padding-box ] ]*
> This is what Gecko has implemented.

Hmm. Here I thought I was just clarifying something that was  
inadvertently omitted. I see now that the working draft [1]  
specifically mentions background-clip as part of what can be part of  
the list of backgrounds, but the editor's draft [2] does not (in fact,  
the editor's draft is more vague there).

It looks as though this was changed in the editor's draft. There,  
'background-color' only takes one value [3], and "[the] background  
color, if present, is painted below all of the other layers." [2]

> If you want to change background-clip to have only a single value
> rather than separate values for each layer, you also need to change:
> * the syntax of background-clip itself

I think you are right. How can 'background-clip' take multiple values,  
if 'background-color' only has one, and is supposed to be drawn on a  
single layer behind all the multiple image layers? Would it use the  
lowest layer's background-clip? Or the outermost of all the background  
clips?

I think there is no real use case for multiple background-colors  
(which I imagine is why it was removed), and I don't really see much  
of one for multiple clippings of background images either. If there is  
only one background-color, then I think there should be only one  
'background-clip'. It would be simpler to just have one clipping, and  
would fit nicer with a drop-shadow proposal I am working on.

> * the rules for the 'background' shorthand

Yes, it currently says that the 'background-origin' value can also  
change the 'background-origin' clip.[4]

[1] http://www.w3.org/TR/2008/WD-css3-background-20080910/#layering
[2] http://dev.w3.org/csswg/css3-background/#layering
[3] http://dev.w3.org/csswg/css3-background/#the-background-color
[4] http://dev.w3.org/csswg/css3-background/#the-background and http://www.w3.org/TR/2008/WD-css3-background-20080910/#the-background
Received on Tuesday, 8 September 2009 16:07:19 GMT

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