Re: CfC: to publish "The srcset attribute" specification as a First Public Working Draft (FPWD)

On Feb 4, at 9:06 PM, Fred Andrews wrote:

> I object to the "The srcset attribute" specification being
> published as a FPWD on the basis that the design makes no attempt
> to meet many of use cases and requirements of the 'Use Cases and
> Requirements for Standardizing Responsive Images' and that the
> editors refuse to consider design changes that would meet these
> use cases.  Further, the srcset specification is related to "The
> picture element" specification and the two can not be considered
> in isolation.
> 
> [ snip ]


> The espoused goal of allowing the user agent to have the option
> to override the authors declarations is not supported in the
> srcset specification design.  The specification does not even
> communicate the image sizes or even the relationship between the
> images sizes - the only information supplied is a set of files
> and the authors hints and if the UA is to ignore the authors
> hints then it is left with just a list of files and could do no
> better than making a random choice.

The relationship between the sizes is communicated by the same criteria used to determine which should be served. It�s not necessary for the UA to know the exact dimensions of the image�if the author has flagged an image source as only being appropriate for high-resolution screens, it is the larger image. Given `sd.jpg 1x, hd.jpg 2x,` it�s clear to the UA which of these will be the larger image. If the concern is that the author will specify these incorrectly, there isn�t much we can do syntactically to avoid that.

The authors suggestions are not to be stripped from the attribute and discarded by the UA�the UA is to determine which of the options presented by the author are most appropriate to the client�s browsing environment.

> 3.5 Relative units
> 
> [ snip ]
> 
> This section mentions use cases that occur post-layout and in
> order for the UA to solved technical challenges such as choosing
> a sharp image it needs to know the images sizes.  To choose the
> lowest or highest resolution images the UA would need to know the
> relationship between them.  The srcset design specification fails
> to supply this information to the UA.

Again, this relationship is defined by the syntax in very simple terms. `1x` is a standard resolution image, `2x` is the source more appropriate for high-resolution displays�and as such, safely assumed to be the larger of the two. Whether one is 200x400 and one is 600x1200 is of no consequence to the UA�it should, if and when it is more appropriate, request the smaller of the two.

> 3.8 User control over sources
> 
> [ snip ]

> The srcset specification gives the UA no information about the
> 'quality' of each image and thus could not support these use
> cases.

See above.

> 4 Requirements
> 
> [ snip]

>  "10. The solution SHOULD afford user agents with the ability to
>  provide a user-settable preference for controlling which source
>  of an image they prefer. For example, preference options could
>  include: 'always lowest resolution', 'always high
>  resolution', 'download high resolution as bandwidth permits',
>  and so on. To be clear, user agents are not required to provide
>  such a user-settable preference, but the solution needs to be
>  designed in such a way that it could be done."
> 
> The srcset specification design does not communicate any
> qualities about the images such as their resolution or size and
> thus the UA does not have the information needed to meet this
> requirement.

See above.

> [ snip ]


> "5 Image candidate strings
> ...
> 
> In this example, a banner that takes half the viewport is
> provided in two versions, one for wide screen and one for narrow
> screens.
> 
> <h1><img alt='The Breakfast Combo'
>          src='banner.jpeg'
>          srcset='banner-HD.jpeg 2x, banner-phone.jpeg 100w, banner-phone-HD.jpeg 100w 2x'></h1>"
> 
> This example is clearly an 'art-direction' use, and not just an
> image resolution choice.  This is inconsistent with the design
> separation between the srcset specification and the picture
> element specification.

Agreed, and for that reason we�ve proposed reducing the scope of the `srcset` syntax to resolution �hinting� only.

> [ snip ]

It seems as though we�re going in circles on this. I might defer to the Chairs on whether this is the appropriate venue for this discussion to continue.


Thanks,
Mat Marquis