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