respimg.md

Adaptive Image Element

Author:

Mat Marquis

Status of this Document

This is an unofficial draft spec, not formally endorsed by the WHATWG. It is suitable only for reviewing the details of the proposed element.

Table of Contents

1. Introduction
2. Implementation Examples
   1. Sample Markup Pattern
   2. Functional Polyfill
3. Example Use Cases
   1. Flexible Layouts
   2. High-Resolution Displays
4. Requirements
5. Prior Discussion

1. Introduction

Our goal is a markup-based means of delivering alternate image sources based on device capabilities, to prevent wasted bandwidth and optimize display for both screen and print.

The idea is to use the video tag’s markup pattern as the inspiration, as it’s specced to allow the use of media queries within attributes on its source elements and reliably displays the markup inside the tag in any browser that doesn’t recognize it. Through use of media attributes we would not only be able to reduce wasteful image requests for the sake of users with smaller displays, but we would be able tailor our images’ resolutions for users with high-res displays or for print.

Much of the surrounding discussion has taken place publicly, in the W3C’s Responsive Images Community Group.

2. Implementation Examples

Any combination of existing media queries can be used to determine the appropriate picture source, through a media attribute on source elements. This is identical to the specced behavior of media attributes on the video’s source elements, as outlined here. Any implementation of the picture tag should allow for the inclusion of fallback markup that is completely ignored by any UA that supports picture, and is only displayed in browsers that do not recognize the new tag. Note that older browsers can be polyfilled (see section 3.2) with behavior similar to a native implementation.

2.1. Sample Markup Pattern

<picture alt="The alt attribute’s content should accurately describe the image represented by all sources, though cropping and zooming of sources may differ.">
	<!-- Matches by default: -->
	<source src="mobile.jpg" /> 
	
	<!-- Overrides the previous source for windows greater than 600px -->
	<source src="medium.jpg" media="min-width: 600px" /> 
	
	<!-- Overrides the previous source for windows greater than 900px -->
	<source src="fullsize.jpg" media="min-width: 900px" /> 
	
	 <!-- Fallback content, only displayed in the event the <picture> tag is unsupported by the browser: --> 
	<img src="mobile.jpg" />
</picture>

2.2. Functional Polyfill

Scott Jehl has put together a JavaScript polyfill that could be used to bring similar behavior to older browsers should <picture> see widespread adoption. As the polyfill is fully dependent on JavaScript, it differs from the behavior of a native implementation in that fallback content is also displayed in the event that JavaScript is unavailable. This ensures a predictable fallback regardless of the presence of JavaScript in older browsers, though a native implementation would have no such dependency on scripting.

3. Example Use Cases

3.1. Flexible Layouts

Serving full-bleed images within a flexible layout or a layout dictated by media queries requires a source image with the largest necessary inherent size and scaling it down through CSS. On smaller displays such as phones and tablets—where bandwidth can be at a premium—this means an exceptionally wasteful request.

While there are currently “responsive images” solutions that deliver smaller images by default and conditionally load a larger image above a certain window size, all of these involve a redundant request and rely fully on JavaScript. On large displays, an image’s src will be prefetched prior to any logic that swaps the image, in many modern browsers. This is further detailed in this post.

<picture alt="Image of a polar bear blinking during a snowstorm.">
	<!-- Matches by default: -->
	<source src="mobile.jpg" /> 
	<source src="medium.jpg" media="min-width: 600px" /> 	
	<source src="fullsize.jpg" media="min-width: 900px" />
	<img src="mobile.jpg" />
</picture>

Assuming a 960px wide window at the time the page is requested and the sample markup pattern in section 3.1, the UA should make a single request for “fullsize.jpg.” Any window/screen smaller than 600px is served “mobile.jpg”, which—as a completely alternate source—could be cropped as well as resized in order to preserve the focus of the image at smaller sizes.

3.2. High-Resolution Displays

High resolution screens such as Apple’s Retina display will require high-resolution images, leaving us with a situation similar to the above: either serving larger, high-resolution images to displays that can’t take advantage of them, or forcing high-density displays to first download a low-resolution image then replace it with a high-resolution image. The latter—the approach currently used on Apple’s website—is far from ideal, and the former is so fraught with concerns that it has recently been adressed in such mainstream publications as the New York Times.

<picture alt="Hero image for new high-resolution device, containing a cringe-worthy portmanteau.">
	<!-- Matches by default: -->
	<source src="standard-res.jpg" /> 	
	<source src="high-res.jpg" media="[-webkit-]min-device-pixel-ratio: 2" />
	<img src="standard-res.jpg" />
</picture>

In this instance, the standard resolution image is served by default and as a fallback in cases where <picture> is unsupported. The high resolution image is served instead only in cases where the pixel-ratio media query matches.

4. Requirements

A conforming user agent must meet the following requirements:

• The appropriate asset MUST be fetched by way of a single request. A change in window size causing the media attribute to match an alternate source SHOULD trigger a request for said source (to be retrieved from the browser cache, if possible).
• As with the <video> and <audio> tags, this solution MUST NOT require any client-side scripting, server-side technologies, or headers to reliably deliver content tailored for the end user’s context.
• Similar to the <video> tag, fallback markup MUST be rendered in any browser that does not recognize the <picture> element. The example in 3.1 uses the “mobile”-sized image as the fallback content, which is the recommended approach: barring the use of a polyfill, the smaller/low-res image should be provided as a fallback to prevent incurring a costly download in contexts that may see no benefit.
• The specification MUST provide at least the same level of accessibility as <img>, with an alt attribute readily accessible to assistive technology.

5. Prior Discussion

How we arrived at <picture> most recently: https://etherpad.mozilla.org/responsive-assets

Common questions and concerns: http://www.w3.org/community/respimg/common-questions-and-concerns/

Prior discussion on W3 mailing lists: http://www.w3.org/Search/Mail/Public/search?type-index=public-html&index-type=t&keywords=picture+element http://lists.w3.org/Archives/Public/public-html/2007Jun/1057.html http://lists.w3.org/Archives/Public/public-html/2011May/0386.html http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2012-February/ (as “Responsive Images”)

MQs for layout and cropping are perfect — those are things UAs can't make themselves.

But MQs should not be used for mere performance optimization. It's an abuse of the tool. MQs should be reserved for choices that UA cannot make better, automatically.

UAs are great at calculating trade-offs between easily quantifiable things like network speed, bandwidth cost, memory availability, cache presence and screen density and zoom level. That should be done by UA, and authors should not be burdened with such non-trivial, but boringly uniform and mechanic selections.

Writing MQ that takes into account all the factors I've listed above is hard. I've tried, and it's a total mess — at least 4 long lines of boilerplate per image. I'm pretty sure authors will not bother (or will make mistakes in the long complicated query), and users will end up with suboptimal choices. Even if MQ language was extended to have first-class or, it would still be pretty tangled boilerplate every author has to copy&paste for every image.

MQs are not future-proof, because when new factors appear, you can't expect all websites on the web to be suddenly updated to take that into account. MQ's can't use new factors before UAs expose them, so the ball is always in UAs court.

Giving optimisation power to the UA could make new factors work immediately without waiting for websites to acknowledge it.

Only UA-controlled optimisation is not enough, so that's why I propose two orthogonal mechanisms.

One huge problem with MQs is that UA is not allowed to override them, even when UA could do better. For example if you say "use low-res image on 3G", and UA has cached a high-res image while it was on WiFi, UA will still be forced to needlessly downgrade the image. In such case MQ-based-optimisation would cause worse experience, and wasted bandwidth.

To be clear: I'm not saying <picture> should not use media queries. MQs are useful. I'm saying MQs should not be involved in decisions made purely for performance reasons.

Performance decisions need another declarative mechanism which allows UA to make the final decision, because we can't expose all factors to authors and expect authors to always make good use of all factors available. Author is wrong person to ask whether I prefer high-res or low-res version of the same image (author doesn't know my mobile contract, doesn't know how many tabs I have open, doesn't know whether I care more about detailed picture or quick loading — but I could configure my UA to know that).

I am dead against doing anything along the lines of "2x". The whole 2x thing is a solution for Apple's very very specific problem of a 2x retina display. It is not appropriate elsewhere, and it is far too specific. Use media queries as a trigger for reacting to device capabilities because media queries are more flexible, and are expanding as required to detect other features such as bandwidth - which are every bit as important as actual pixel dimensions, if not more so.

Could you replace the "Scott Jehl" link with one to the actual polyfill? That would be a lot more useful.

Was there prior discussion about making the <img> child element also the one matching by default? In your examples, default <source> and the fallback both have the same src, so reducing that duplication would be nice.

"we could selectively serve high-resolution images to high-density displays only when on a 4G-or-better connection"

Isn't the browser in a better position than the author to determine whether the user would want the high-resolution image immediately? It seems to me that it's better to let the author say "these are the images I have, they have these properties" and let the browser choose the version it wants, than to force authors to write rules choosing between viewport size, device pixel size, zoom level, bandwidth, etc, and expect them to get it right. From the examples I've seen, the actual behavior would be completely backwards in some situations (e.g. zoom in on desktop) or even result in no image at all (e.g. a list of min-width rules and the user has a smaller width than what the author cared to list).

We have been in a situation before where browsers had to lie about their properties to get sites to give them content with better user experience (e.g. mobiles moved from "handheld" media to "screen" media because the user experience was horrible on sites if you said you were a "handheld").