RE: <img src="..." defer>

This is exactly the problem we're trying to solve with Resource Priorities. I also prefer reusing the "defer" attribute, specified on any HTML element capable of fetching a resource, to mean that the user agent should delay downloading the resource.

I have been working on a spec proposal for Resource Priorities that closely aligns with the ideas below. We have been scheduling conference call discussions on all of the new Web Perf WG specifications. I recommend we schedule time to discuss this topic for this Wednesday's conference call.

Thanks,
Jatinder

From: Jake Archibald [mailto:jakearchibald@google.com]
Sent: Sunday, April 14, 2013 10:53 AM
To: public-web-perf@w3.org
Subject: <img src="..." defer>

Hi,

I know this group is looking at resource priorities, but I'm interested in taking it a big further for images, and perhaps other visual media such as video.

# Problems I'd like to solve:

Developers are using hacks or ending up with unnecessary downloads when they use JavaScript to change the src of an image, but want to provide a non-JS fallback (see https://www.w3.org/Bugs/Public/show_bug.cgi?id=17842). Although this particular case will be solved with something like srcset, there are other cases like conditionally using an image format with limited support.

Developers are using hacks to lazy-load images that are likely to be outside the viewport initially. These hacks use heavy scroll listeners and don't take the connection type into account.

Images within elements that are never shown are downloaded.

# "defer" behaviour:

Images with 'defer' MUST NOT download while they aren't in a document, or their calculated 'display' style property is 'none'.

The download state of images with 'defer' MUST NOT delay the 'load' event of the window.

Images with 'defer' MAY download once they are in the document, and their calculated 'display' style property is not 'none'.

Images with 'defer' MUST download once they are in the document, and their calculated 'display' style property is not 'none', and any part of the image would be visible within the viewport. If visibility within the viewport is uncertain (due to element size determined by image dimensions), the image MUST download.

There's been a lot of discussion on https://www.w3.org/Bugs/Public/show_bug.cgi?id=17842, I'll try and sum up the pros & cons but will ping Ilya to add anything I may have missed.

# Pros
* Imagery in hidden elements that may never be seen by the user aren't downloaded
* JS can modify the src of images before their original src downloads, by initially hiding the image with CSS
* The browser gets more freedom in regards to image downloading, allowing it to do the best thing given the user's connection type & interaction. Eg:
** If making a connection is cheap & quick (eg, fast wifi), the browser may delay downloading imagery until it's in the viewport, which can avoid unnecessary downloads way below the fold
** If making a connection is expensive & slow (eg, 3g), the browser may download all imagery that isn't calculated display:none to avoid waking the radio up later and using up battery

# Cons
* When creating images with js, you'd need to set .defer to true before setting .src, else the image would download straight away

I don't see this as a big problem, devs know the ordering matters, you already have to set src after load events

* Deferred images cannot be downloaded early by preloaders unless they calculate page styles & images that are at the top of the page may download later as they need to wait for style calculation to know if they should download or not

Not a big deal. Images are low priority downloads anyway compared to JS & CSS so waiting for layout isn't a problem. Also, primary imagery shouldn't use this attribute.

* Firing the load event before all images download may skew metrics

By putting 'defer' on an image you're saying it isn't required to consider the page loaded. I don't think this is an issue.

* Lazy-loading isn't good on mobile connections

The browser isn't required to lazy-load. It can make the choice based on the connection, which is far better than the hacky JavaScript developers are using to currently perform lazy-loading.

* Developer can't indicate they'd rather have aggressive lazy-loading, to save CDN costs

Feature, not a bug. Would rather the browser did what's best for the connection type. The developer could always display:none the images, and use a scroll listener to show them as they come into view, but I wouldn't want to make that any easier.

Cheers,
Jake.