Responsive IMGs — Part 1

September 27th, 2011 by Jason Grigsby |
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In my post “Where are the Mobile First Responsive Web Designs”, I noted that one of the first things I look for when trying to determine whether or not a responsive web design is “mobile first” is whether or not it has a strategy for handling the IMG tag.

A recent Smashing Magazine round up of responsive web design techniques included several new approaches for handling IMG tags which makes it the perfect time to dig into this problem and the potential solutions in more depth.

Why IMG Tags Suck for Responsive Web Design

If you want your site to load as quickly as possible, you do want to deliver larger files than are needed. Many responsive web design sites provide mobile devices images at sizes appropriate for desktop and ask the mobile device to resize the image.

In my research, I found nearly 80% decrease in file size by delivering images at the actual size they were going to be used on a mobile device.

So what’s the problem with the IMG element in responsive designs? Unlike CSS images which can provide different source files based on screen resolution using media queries, IMGs have a single source attribute.

What are Responsive IMGs?

Responsive IMGs are images delivered using the HTML IMG tag that come from different sources depending the screen size. There are many different techniques for accomplishing Responsive IMGs.

As far as I can tell, Scott Jehl first coined the phrase Responsive Images to describe a javascript solution to the img source problem. He also referred to Responsive IMGs as a general term recently so I’m hopeful he doesn’t mind the fact that I’m extending his definition to describe any technique that attempts to provide images at an appropriate size for a responsive design.

Responsive IMGs Challenges

There are some common challenges that any Responsive IMG technique will face. As we review the various techniques that have been proposed, we need to keep these challenges in mind.

Minimum Bar: Start with Mobile, No Extra Downloads

Scott Jehl set a minimum bar for Responsive IMGs by stating they must do the following:

  1. Start with mobile img
  2. Upgrade to larger size without downloading both

Both of these are worthy and necessary goals.

The First Page Load Problem

Any solution that relies on client-side scripting to make a decision about what image source to display will suffer from a first page load problem. The first time someone visits a site, the server won’t know what size image to provide.

responsive images 1

Image from Bryan Rieger’s Muddling Through the Mobile Web presentation, photo by wscullin, licensed under Creative Commons.

If javascript is added that determines what image size is appropriate, then this information can be retained for the user session via cookies or similar techniques. In theory, on subsequent requests the server can make a decision about what size image to include in IMG tag.

FWIW, the speed of first load is a big deal. The speed of a person’s first experience can dictate their impression of a product and company. Google, Yahoo and others have talked about how minor speed differences makes a big difference in usage of their products.

Rendering Race Conditions

Techniques that rely on adjusting the image source attribute via javascript need to make sure that the modification happens before the image requests start.

Browser makers have done a lot of work to download as many assets as possible at the same time. Usually this is a good thing. But in the case of responsive imgs, the javascript needs to evaluate what size image to retrieve before any image requests start.

A lot of earlier work was done using dynamic base tags. This worked when the javascript was inline in the head tag, but failed to prevent images from downloading twice when an external javascript file is used.

The upshot is that nearly every client side technique requires deep understanding of the order in which different browsers process and download assets. Or more realistically, each approach needs to be tested extensively.

Content Delivery Networks and Caching

When you deliver different size images at the same url, you can run into problems with CDNs and other caching at the edge of the network. If the first person to request an image is on a mobile phone, people who follow via the same CDN or cache will also see the mobile-optimized image even if they are on desktop unless consider CDNs in your strategy.

Future Friendly Responsive IMGs

If we accept that the “quantity and diversity of connected devices—many of which we haven’t imagined yet—will explode”, then we need to consider look for solutions that are future friendly. In addition to the current experimentation, we need to start thinking about what a long term solution might look like.

For example, many of the early solutions for Responsive IMGs consist of two size images: one for desktop and one for mobile screen sizes. Will two image sizes really suffice for all of the devices that are coming?

Also, a lot of the solutions right now tackle one part of the problem. They may tackle the client side changes to switch the image source, but leave as an exercise for the developer to figure out how image resizing will be handled. For shared libraries, limited scope makes sense.

But as we look at what systems will need to do to be successful in the future, we need to think about what we want out of both the server and client side.

Here are some of the things that I think a future friendly technique will need to consider:

  • Support arbitrary image resizing — We cannot anticipate what screen sizes may be coming. We need systems that handle image resizing automatically and support any arbitrary size needed for a particular page.
  • Art direction can override automatic resizing — Not every image can be resized without losing the meaning of the image. Sometimes cropping an image may work better than resizing it. Automatic tools need to easily support manual override.

unreadable image

  • Support for higher resolution displays — What do we do with the iPhone 4’s retina displayand other devices sharing similar high resolution screens? It is an open question about whether we should deliver higher resolution images to those devices given the performance hit that will occur if the person is on a slow connection.But regardless of how we chose to handle it right now, it is clear that the trend towards more pixels per inch on displays is not going away. If anything, we’re seeing indicators that higher density will soon be available on desktop displays as well.This means that our current definition of what is a large image for web is probably too small for future devices. With that in mind, it probably makes sense for systems to accept the highest resolution image possible—even if that resolution isn’t currently being used—so that when new devices become available the high resolution source is already available and hasn’t been lost.
  • Connection speed should be part of the criteria — We can be much smarter about the size of the image we deliver if we can tell something about the network connection. We need an easier way to get at this information.
  • A replacement for the IMG tag? — All of the responsive image solutions are attempting to deal with the fact that the image tag has only a single source. There have been various proposals recently to take a new look at what the tag should be and see if we can find a long term replacement.

That’s my short list. What would you add?

In part 2, I’ll take a closer look at the current alternatives for responsive imgs and which ones hold the most promise.

Lynda DamiataResponsive IMGs — Part 1