Cloud Four Blog

Technical notes, War stories and anecdotes

The End of Mobile Portland

In December 2007, Lyza, Aileen, John and I decided to start Mobile Portland so we would have place to talk about mobile.

After eight years, Mobile Portland is coming to an end. Tonight is the final meeting.

Over the years we’ve had some amazing speakers and topics. I’m proud of the quality of the talks and the community we built.

We also inadvertently started a worldwide open device lab movement. It’s been amazing to see device labs spread and know they started in Portland.

I’d like to thank everyone who attended a meeting and those who helped out in any way. Every small contribution lifted a huge burden off the shoulders of frantic organizers.

I want to extend a special thank you to my co-founders at Cloud Four for helping get Mobile Portland off the ground and funding it; Matt Gifford for organizing so many meetings; Seth Shikora for recording nearly every meeting we’ve held; and to Elia Freedman, Dylan Boyd and Rob Mills for being the best board members I could have asked for.

Tonight’s meeting

But before Mobile Portland rides off into the sunset, we have one final meeting tonight, and it is going to be the best one yet!

I can’t think of a better speaker and a more fitting topic than Josh Clark talking about Magical UX and the Internet of Things. This is the next frontier of technology and mobile plays a bit part in it.

This will also be our largest meeting ever. We’ve had to create a waitlist for the first time so if you’ve already RSVP’d and are unable to make it tonight, please update your RSVP so people on the waitlist can attend.

What’s next?

Since we announced the end of Mobile Portland, people keep asking me two questions. First, “Why end Mobile Portland when there is still a lot of interest?”

Because eight years is a long time to do anything and instead of the group gradually winding down and losing relevance, we made the decision to go out on top.

The second, inevitable question is, “What’s next?”

I’m pleased to say that I can finally answer that question. Let me tell you about Responsive Field Day.

Responsive Images 101, Part 5: Sizes

When we last left our intrepid web developers, they had discovered the power of srcset width descriptors, only to be faced with a new challenge—the browser only knows the size of the viewport when it begins downloading images.

Now, it is time to meet the hero of our story: the sizes attribute.


Sizes attribute is required!

The sizes attribute is required any time you use srcset width descriptors.

In fact, sizes only makes sense if you’re using the width descriptors. If you’re using the display density descriptors, you don’t need the sizes attribute. The browser won’t know what to do with it.

Sizes syntax

Out of all the new responsive images standards, sizes was the hardest one for me to wrap my head around at first.

Sizes syntax repeated below

Like srcset, the sizes attribute contains a comma-separated list. This comma-separated list describes the size of the image in relation to the viewport.

I want to repeat that point because it is the key to understanding sizes.

We’re telling the browser what size the image will be in relation to the size of the viewport. And we can tell the browser how that relationship changes as the size of the viewport changes.

<img src="cat.jpg" alt="cat"
  srcset="cat-160.jpg 160w, cat-320.jpg 320w, cat-640.jpg 640w, cat-1280.jpg 1280w"
  sizes="(max-width: 480px) 100vw, (max-width: 900px) 33vw, 254px">

Like srcset, each comma-separated item contains two values separated by a space.

Media conditions

The first value is a media condition. A media condition is similar to a media query, but not as full featured. For example, you can’t do things like ‘@media screen’, but you can do everything else you would likely want to do in sizes.

Most commonly, your media condition is going to be a something like ‘(max-width: 480px)’ or maybe ‘(min-width: 480px)’.


The second value in each comma-separated item is a length. This length is often expressed using the viewport width (vw) unit.

There’s a good chance you haven’t seen vw units before. They are fairly new, but have wide support in current browsers.

Each vw unit represents 1% of the viewport width, which is a fancy way of saying that 100vw is 100% of the viewport width and 33vw is 33% of the viewport width.

The length doesn’t have to be expressed as a viewport width unit. It can be any length including absolute and relative length. You can even use CSS calc() to do things like auto-calculate margins dynamically.

How does the browser select the correct sizes value?

When the browser starts through the comma-separated list of values, it grabs the first value where the media condition passes.

Take another look at our sample markup and the order in the sizes attribute:

<img src="cat.jpg" alt="cat"
  srcset="cat-160.jpg 160w, cat-320.jpg 320w, cat-640.jpg 640w, cat-1280.jpg 1280w"
  sizes="(max-width: 480px) 100vw, (max-width: 900px) 33vw, 254px">

If we translated this into a bulleted list of instructions, it might look like this:

  • (max-width: 480px) 100vw — If the viewport is 480 pixels wide or smaller, the image will be 100% of the viewport width.

  • (max-width: 900px) 33vw — If the viewport is 480 pixels wide or smaller, this rule will never be reached because of the previous media condition. Ergo, if this rule effectively says that if the viewport is 481 pixels wide to 900px, that the image will be 33% of the viewport width.

  • 254px — When there is no media condition listed, the length is assumed to be a default value used when none of the other media conditions are met. In this case, we have media conditions covering viewports up to 900 pixels. Therefore, from 901 pixels wide to infinity, the image will be 254 pixels wide.

To help you visualize how this might work in the real world, I created a little video that looks as how the values might change as the viewport width increased on the Walmart Grocery site.

NOTE: As of the time of publication, the Walmart Grocery site was not using srcset and sizes. This is hypothetical markup. If you want to see srcset and sizes in action, take a look at The Guardian which recently switched to using srcset and sizes.

But what about separation of content and presentation?

I’ve seen many complaints about the new responsive images specification. Most amount to either complaints about complexity that ignore the fact that images on the web are inherently complex3 or some variation of WWIC.

But the one complaint I have a tremendous amount of sympathy for is the idea that we now have presentation information—the size of the image—in the markup. I doubt there was anyone involved in the responsive images standards process who didn’t share this concern at some point.

Unfortunately, it is unavoidable. As discussed in Part 4, the browser starts downloading image sources before the size of the image in the page is known.

The only way to support the pre-loader and make sure the right source gets downloaded is to provide some information about the size of the image in the markup.

Is the pre-loader worth it?

If you’re like me, you may have found yourself wondering whether the pre-loader was worth all of the problems it causes?

Yes. Yes, it is.


Andy Davies wrote about how Google saw a 20% and Firefox a 19% increase in average page speed after implementing the pre-loader. Steve Souders thinks that “preloading is the single biggest performance improvement browsers have ever made.”

We can’t simply throw out that web performance boon in favor of responsive images.

Therefore, we have to find a compromise. The sizes attribute is that compromise. It provides just enough information for the browser to do its job.

Srcset and sizes = Smart browsers

Srcset and sizes provide all of the functionality you need for the resolution switching use case. They give the browser just enough information to allow it to make smart decisions.

Dog with glasses

But what happens when you need more control? What about art direction?

Next Tuesday, Responsive Images 101, Part 6: The Picture Element.

Responsive Images 101 Series
  1. Definitions
  2. Img Required
  3. Srcset Display Density
  4. Srcset Width Descriptors
  5. Sizes
  6. Picture Element
  7. Type
  8. CSS Responsive Images
  9. Image breakpoints

  1. How quickly we forget web-safe colors, Lynda Weinman‘s multiple books on web graphics, and the way different images formats compress. And it’s not getting any simpler. Images on the web are inherently complex.
  2. Super hero by Ashley Rose. Dog Intelligence by Alice Jamieson. Licensed under Creative Commons.

Responsive Images 101, Part 4: Srcset Width Descriptors

In Part 3, we looked at display density descriptors and concluded that they are great for fixed width images, but are insufficient for flexible images.

Flexible images is where srcset’s width descriptors shine.

Width descriptors

The syntax for width descriptors is similar to that of display density descriptors. The value of the srcset attribute is a comma-separated list of image sources and descriptors.

Srcset syntax with width descriptors. Repeated below.

The difference is that instead of having 1x, 2x, and other values representing the density, we now list the width of the image source such as 320w, 480w, etc.

<img src="cat.jpg" alt="cat" 
  srcset="cat-160.jpg 160w, cat-320.jpg 320w, cat-640.jpg 640w, cat-1280.jpg 1280w">

The width of the image source can cause some confusion. Width descriptors are looking for the resolution of the source file.

In other words, if you open the image in an image editor, what does it say the resolution is? Grab the width and put it in the srcset attribute.

The browser picks the best source?

When you use width descriptors, you’re providing the browser with a list of images and their true widths so that it can select the best source. How does the browser do that?

Your first instinct is probably to say that the browser looks at the size of the element in the page and compares it to the list of source sizes. That makes sense, but it isn’t how the selection work.

See, when a browser starts downloading images, it often doesn’t know the size of the image in the page.

Browser pre-loader and speculative asset downloading

If you look at a timeline of how a browser renders a page, you’ll notice something striking. Chrome timeline

Shortly after the browser downloads the HTML, it requests CSS and JavaScript. But before the CSS and JavaScript is done loading, the browser starts downloading images.

Because neither the CSS nor JavaScript is complete, the browser is downloading images without knowing what the layout of the page will be. And without knowing the layout, it doesn’t know what size the image element will be.

BTW, this pre-loading behavior is why we can’t solve inline responsive images using CSS or JavaScript. They aren’t available when the browser starts downloading.

The only thing that the browser does know is the size of the viewport. Once we move past display density descriptors, everything hinges on the size of the viewport.

Why does this matter?

The viewport can be a poor substitute for the actual size of the image. Take the images on Walmart’s Grocery site:

Small screen version of showing that images are nearly full width of page.

On narrow viewports, the images are nearly the same size as the viewport width. They are certainly close enough to work.

Wider screens, however, are a different matter:

Wide screen version of showing how images are much smaller than the viewport.

In the second example, the viewport is 1540px wide, but the images are only 254px wide. Knowing the size of the viewport won’t tell the browser enough information to be able to select the right image source.

Sizes to the rescue!

How do we tell the browser about the size of the image in the page so that it can download the right source from our srcset list? Use use the sizes attribute!

Read more in Part 5: Sizes.

Responsive Images 101
  1. Definitions
  2. Img Required
  3. Srcset Display Density
  4. Srcset Width Descriptors
  5. Sizes
  6. Picture Element
  7. Type
  8. CSS Responsive Images
  9. Image breakpoints

Responsive Images 101, Part 3: Srcset Display Density

Ever since Apple launched a retina display on the iPhone 4, web designers have been looking for a way to handle high density displays. That is where srcset and its display density descriptors come in.

Resolution switching uses srcset

As a reminder, display density is a resolution switching use case. And when we’re solving for resolution switching, we want to use srcset.1

The reason why we want to use srcset is because it gives the browser choice. When we use the media attribute provided by the <picture> element, we’re dictating to the browser what image it must use. That makes sense for art direction.

But when it comes to resolution switching, the browser knows best what image will work. It can make decisions based on factors that we’re not privy to such as network conditions or user preference.

So unless we are doing art direction, we should strive to give the browser options and let it make smart decisions.

Display density descriptors

The syntax for display density is fairly straight forward:

srcset example repeated below

The srcset attribute is added to an <img> element. The value of srcset contains a comma-separated list. Each item in the list contains the path to an image and the density of that image provided as a multiple (e.g., 1x, 2x, 3x…).

<img src="cat.jpg" alt="cat" srcset="cat.jpg, cat-2X.jpg 2x">

The display density values—the 1x, 2x, etc.—are referred to as display density descriptors. If a display density descriptor isn’t provided, it is assumed to be 1x.

That seems easy…

It is easy—assuming all you care about is display density. I have my doubts about how often that will happen.

Let’s take a look at the Apple Watch image from Part 1:

Apple Watch Hero Image

As mentioned previously, the image is 5144×1698 pixels and 398K in its 2x incarnation. There is also a 1x version. Let’s compare them to the size that would make sense for a single density, Blackberry Curve 9310:

Image Width Height File Size
2x large 5144 1698 398K
1x large 2572 849 256K
320px, Single Density 320 106 8K

For the final row in the table, I resized the image to 320px wide and saved as a JPEG in order to estimate what it would be.

It should be obvious that two sizes of an image aren’t sufficient. Sure, we could start at 320 as 1x and then rewrite our markup to look something like this:

<img srcset="cat.jpg, cat-2X.jpg 2x, cat-3x.jpg 3x, […], cat-16x.jpg 16x">

That will get us from 320px to the 5144px of the largest image, but it seems insane to me.

And this highlights another reason why I find the display density descriptors to be less desirable than other solutions. I don’t have any interest in keeping track of all of the different display densities available.

Do we want to provide 1x, 1.5x, 2x, 3x variants? What about accounting for things like the iPhone 6 Plus’s downsampling?

Not to mention what happens when you start working with flexible images. Now you have multiple densities at multiple image breakpoints. And sometimes you’re repeating your image sources because 2x at a small size could be the same as 1x resolution at a larger image breakpoint.

It gets messy quickly.

Display density descriptors are best for fixed width images

The moment you move beyond providing alternate densities of a fixed width img element, the display density descriptor becomes unwieldy and inadequate to the task.

What do you need instead? Part 4: Srcset Width Descriptors.

Responsive Images 101 Series
  1. Definitions
  2. Img Required
  3. Srcset Display Density
  4. Srcset Width Descriptors
  5. Sizes
  6. Picture Element
  7. Type
  8. CSS Responsive Images
  9. Image breakpoints

  1. Unless we’re providing different image formats which we will cover later.

Responsive Images 101, Part 2: Img Required

One of the main reasons why we need solutions for responsive images is because the <img> element is insufficient. It only has one src and we need multiple sources.

Given that fact, it may be surprising that we’re going to start by talking about the <img> element instead of the new, shiny toys like <picture> and srcset.

No matter what responsive images solution you use, <img> is required.

The <img> element is critical for all of the inline responsive images solutions. The way I like to think about it is that the img is a box into which all of the responsive image rules are added and applied.1


You can use JavaScript to watch for changes in the currentSrc of the img element. Here is an incredibly simple script that watches for changes and writes them to the page2:

(function() {
  var currentSrc, oldSrc, imgEl;
  var showPicSrc = function() {
    oldSrc     = currentSrc;
    imgEl      = document.getElementById('picimg');
    currentSrc = imgEl.currentSrc || imgEl.src;
    if (typeof oldSrc === 'undefined' || oldSrc !== currentSrc) {
      document.getElementById('logger').innerHTML = currentSrc;
  // You may wish to debounce resize if you have performance concerns
  window.addEventListener('resize', showPicSrc);
  window.addEventListener('load', showPicSrc);

You can see this in action on this demo page3. Resize the browser to see the currentsrc change.

Why does this matter?

The fact that the <img> will always show the current source means that any existing JavaScript that interacts with the <img> element will continue to work as expected.

(Not to mention the decades worth of code that browser-makers have written to handle images correctly.)

As Eric Portis puts it, “we’re progressively enhancing the img” when we use the new responsive images standards.

Do you need more than img?

There are some scenarios where the img element alone might be enough:

  • A fixed width, single density web page — If you don’t have a responsive design and aren’t worrying about “retina” displays, then the img element is all you need.

  • Small differences in file size — For some images there isn’t much difference between the file size of the largest and smallest variants. We’ve seen this with badges, icons and other small images that don’t change much as the viewport changes. If there isn’t much difference in file size, a single img source may suffice.

  • Using vector-based images like SVG — If you’re using a vector-based image format like SVG, the image can scale without providing multiple sources. In that case, you may be able to link directly to the SVG as the single source for the img.

    This depends, of course, on whether the browsers you support also support SVG. You may be better off using the picture element to provide alternative image formats as fallbacks—something I’ll cover later in this series.

What if you want to support high-density displays?

If you want to support high-density displays, then we need to supersize our <img> element. Learn how in Part 3 in this series: Srcset Display Density.

Responsive Images 101 Series
  1. Definitions
  2. Img Required
  3. Srcset Display Density
  4. Srcset Width Descriptors
  5. Sizes
  6. Picture Element
  7. Type
  8. CSS Responsive Images
  9. Image breakpoints

  1. Box icon made by Daniel Bruce from and is licensed under CC BY 3.0
  2. Thanks to Lyza and Erik for helping with the JavaScript.
  3. Demo page forked from Google Chrome Team picture element example that was used in Pearl Chen‘s great article on Built-in Browser Support for Responsive Images.