[whatwg] RWD Heaven: if browsers reported device capabilities in a request header

[Boris Zbarsky]

On 2/6/12 3:10 PM, Matthew Wilcox wrote:
>
> On 6 Feb 2012, at 18:58, Boris Zbarsky wrote:
>
>> On 2/6/12 1:55 PM, Irakli Nadareishvili wrote:
>>> Many thanks to everybody who has responded and for a lively and a productive discussion!
>>>
>>> Quick clarification: the proposal is to include *device* capabilities in the HTTP headers, so when we say "screen width and height" we mean device screen width and height which is constant
>>
>> Again, it's not constant in the terms that the page sees, which are CSS pixels, not device pixels.
>
> Media queries and explicit widths make this statement false, in a practical way.

How so, exactly?

> We adjust for physical pixels quite happily by specifying a width on the image.

The width you specify is in CSS pixels.  If the device is running at 
200dpi, and you specify a 100px width for your image, it'll make the 
image 200 device pixels wide.

> They're then down sampled into physical pixels even though CSS is using CSS pixels.

You assume "downsampled", but it can just as easily be "upsampled".

But again, my point was that the "width of the screen" in CSS pixels is 
not an invariant.  If you set your image to be "100px" wide, then on the 
very same device it can be all sorts of fractions of the width of the 
screen, depending on UA settings.

-Boris