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Re: [css3-page][css3-mediaqueries] Page size conflicts

From: Brad Kemper <brad.kemper@gmail.com>
Date: Mon, 20 Feb 2012 14:44:18 -0800
Message-Id: <37216BB8-6AD4-48CA-846B-CD136E10C8B7@gmail.com>
Cc: "www-style@w3.org" <www-style@w3.org>
To: Simon Sapin <simon.sapin@kozea.fr>
On Feb 20, 2012, at 1:20 PM, Simon Sapin <simon.sapin@kozea.fr> wrote:

> Le 20/02/2012 20:10, Brad Kemper a écrit :
>>> Nice. What about this one?
>>>   @page :left {
>>>     size: 4in 6in;
>>>   }
>>>   @page :right {
>>>     size: 6in 4in;
>>>   }
>>>   @media (min-width: 5in) {
>>>     div { background: blue; }
>>>   }
>>>   @media (max-width: 5in) {
>>>     div { background: red; }
>>>   }
>>> Assuming there is a div with a page break, what color is its background? (And what is the computed value of background-color?)
>> Following the same logic as above, The left page is 4in wide, and the right page is 6in wide. So on the left the div is red, and on the right the div is blue.
>> Kind of messes with box-decoration-break though, if you had two different background images instead of just background-color.
> Hi,
> ***Note***: This whole message is based on the idea that results of media queries could vary across pages (if pages have different sizes). However, as Øyvind pointed out, media query expression never depend of the result of applying a stylesheet:
> http://lists.w3.org/Archives/Public/www-style/2012Feb/0976.html

I saw that, but my premise was that @page could carve itself out an exception, without creating any circular logic.  

> Initially I didn’t see how this could make sense but I see after reading on regions. The cascade gives a set of computed styles for an element. That element may be fragmented into multiple boxes for lines, pages or regions. Under some conditions, some of these fragments can get different styles. As you say, the @region rule is such a case. But now that I think of it, this is already the case with ::first-line and ::first-letter.
> But note that both Regions and Selectors (for ::first-line and ::first-letter) defined a limited set of properties that can be used in such contexts.

So does css3-page. 

> For example, it would not make sense for fragments of the same element to have different values for the 'display' property.
> While it would be technically possible to have the same kind of behavior for page breaks (different style across pages like above) I think it is a bad idea.

It is a core feature of @page and that entire spec. You can have different styles (within limits) for the first page, left page, right page, and arbitrary named pages, than what you have in general for all other pages. 

> For example:
> 1. @media all and (color) { #foo { float: left }}
> 2. @media all and (min-width: 15cm) { #foo { float: left }}
> The result of the media query 1 does not vary across pages, but 2 does. I think that 'float' should not change across pages, so it would not be allowed in 2. Should we forbid it in 1 too or keep the rules (seemingly) inconsistent?

I don't understand why #2 should be prohibited. It seems like a reasonable thing to me. See below for more (where I say, "Ah, I see")...

>>> And even without page breaks, we only know which page a box is on when doing layout, but the cascade is done much earlier than that.
>> Is that a problem if done as I describe? Seems no worse than with regions, where you wouldn't know what region an item is in until you do layout.
> Indeed. I haven’t implemented any of regions, first-line of first-letter in WeasyPrint yet so I still have the cascade done before the layout starts. Things are more complex with regions, but still doable.
>>> I think the only sensible thing to allow in such media query rules is @page, except for the 'size' property.
>> Sorry, but my brain doesn't quite parse that sentence. Can you restate it?
> ***Again***, any of this message does not apply. (See the start of this message.) Media queries of the page size use the UA’s default page size, not any stylesheet result.

I think that can and should change in the way I've described, and it can be detailed in the paged media spec, so it would apply.

> Sorry I was not clear. Let me rephrase with examples:


> If we *want* it to apply, Media Queries would need to be changed.
> Examples:
> 1. @media all and (min-width: 15cm) { @page { margin: 3cm }}
> 2. @media all and (min-width: 15cm) { @page { size: 21cm 29.7cm }}
> 3. @media all and (min-width: 15cm) { #foo { color: red }}
> 4. @media all and (min-width: 15cm) { #foo { float: left }}
> 5. @media all and (color) { #foo { float: left }}
> 6. @media all and (color) { @page { size: A4 }}
> * 1 is fine and could be allowed.
> * It was agreed earlier in this thread that 2 has a conflict and should just be forbidden/ignored.

Yes, it was, by Boris and fantasai, rather quickly. As I consider this more, I'm not sure of why it needs to be like that though. Wouldn't it be just as good or better to say that an @size inside a media query does not affect the results of any media queries? That seems simpler to me, with no big downside. Is there real value in media queries ever basing their results on an @size? What am I missing?

> * As explained above, 3 could work but I think it is a bad idea.

I disagree with that. Aren't you then completely gutting any media query that is based on a width of any sort? What is left that you would allow for that query, if you don't allow color changes? Or are you saying width-based query only wouldn't apply if there was more than one page size? Would you always know that at the time of the cascade.mMaybe I am misunderstanding you? 

> It should be forbidden to be consistent with:
> * 4 should be forbidden as (I think) it does not make sense for an element to be only partly floating. If not 'float', some properties will end up forbidden like in regions.

Ah, I see. It is a interesting problem. I think I would say you could still allow float in width-based media queries, but if a float was started on one page it could continue on the next page, regardless of what the media query for that second page had today about it. 

> * 5 and 6 does not vary across pages, no problem
> To sum up, I think that anything in a media query should only be allowed if either:
> * It is in the @page context (or nested in @page, like margin boxes), but is not the 'size' property (as in 1)

So, in #1, you would allow the margin property to be valid because it was inside @page, but otherwise not? But then you couldn't use that query for continuous media! Or if you did allow it for continuous media only, then it would be really unexpected to have the styles disappear when printed, even when the query otherwise matched and all the pages were the same size.

> * The media query expression does use the page size and thus can not vary across pages (as in 5 and 6)

Did you mean "doesn't"? If not, I'm more confused.
Received on Monday, 20 February 2012 22:44:42 GMT

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