Cloud Four Blog

Technical notes, War stories and anecdotes

Listen to WalmartLabs on the RWD Podcast

WalmartLabs LogoWe had the great fortune to help WalmartLabs convert their sites to responsive design, and it is always nice to see our clients get recognized for their hard work.

Mini Kurhan and Olawale Oladunni, two designers at WalmartLabs, were recently interviewed by Ethan Marcotte and Karen McGrane for the Responsive Web Design Podcast.

I highly recommend giving the interview a listen. There is also a transcript of the interview if you prefer.

It was great hearing Mini and Ola talking about the challenges Walmart faced and how they overcame them. It was a massive effort to convert the sites to responsive design and the Walmart team did a fantastic job. We had a blast working with them.

If you want to learn more about the Walmart redesign, particularly when it comes to responsive images, check out Mini and Ola’s presentation from Responsive Field Day.

A Responsive Guide to Type Sizing

When starting new projects, a CSS builder’s initial concerns tend to involve typography. Setting a typographic foundation with the right mixture of ingredients can form something solid enough to support many other building blocks of design. It’s an approach that makes structural sense.

Proportions are a key ingredient to the mixture. Calibrating your type proportions for a balance of aesthetics and order can be an obsessive undertaking. It’s a challenge getting proportions right for a given screen size, let alone any possible screen size. This process can be less challenging — even for responsive designs — if you use a modular scale and let math do the work for you.

Using a modular scale for typographic proportions

In his detailed article on the subject, Tim Brown demonstrates using a modular scale for composing type. He explains how compositions can benefit from using numbers that relate to each other in a meaningful way:

Modular scales[…] help us achieve a visual harmony not found in compositions that use arbitrary, conventional, or easily divisible numbers.

This modulation-based approach is particularly useful for determining type sizes. Selecting sizes using a ratio (instead of guessing) yields a more harmonious range of proportions.

With tools like this indispensable calculator, you can see how different bases and ratios affect your scale. These are the variables in your scale’s equation. The value produced by multiplying or dividing these variables is a step on your scale. Repeating the operation on each resulting value forms a new step:

Step Math Result
+3  40 × 2 80   
+2  20 × 2 40   
+1  10 × 2 20   
 0  none 10   
−1  10 ÷ 2  5   
−2   5 ÷ 2  2.5 
−3 2.5 ÷ 2  1.25
Example with a base of 10 and a ratio of 2

You can also associate each step in the sequence with the number of times the ratio has been multiplied or divided:

Step Math Result
+3 10 × 2 × 2 × 2 80   
+2 10 × 2 × 2     40   
+1 10 × 2         20   
 0 none 10   
−1 10 ÷ 2          5   
−2 10 ÷ 2 ÷ 2      2.5 
−3 10 ÷ 2 ÷ 2 ÷ 2  1.25

Experimenting with base and ratio parameters is a quick way to gauge how much proportional contrast is right for your project. Compare the following scales:

Due to their ratios, these scales have wildly differing degrees of change between each step. The “golden section” ratio yields much more change than the “minor third”. Consider how much change you’re likely to need, and how it correlates with varying screen sizes and content hierarchy.

Comparing two scale ratios

Scales with the ratios of 1.618 and 1.2 compared on

The balance of typographic contrast in scale is dependent on screen size. A scale that works well on a large screen may have too much contrast for a small screen (and feel obtrusive). Likewise, a scale that is harmonious on a small screen may be too subtle when viewed on a large screen.

One solution to this conundrum is to create a scale intended for small screens that is also capable of adapting for large ones. The first thing to consider when creating that scale is which ratio to use.

Step Font size Result
 0   1.0em
+1   1.2em
+2  1.44em
+3 1.728em
+4 2.074em
+5 2.488em
The minor third ratio (1.2) with a base of 1em

The minor third ratio of 1.2 yields a gradually changing range of sizes. They all feel reasonable for small screens, and are distinct enough for levels of typographic hierarchy. At step #5, the resulting size can still be used while retaining a reasonable number of characters per line. With these attributes, this ratio feels like a good selection for small screen proportions.1

Applying your scale from the ground up

After figuring out a scale that works well for your project, you’ll probably want to put it into practice. CSS offers a few options:

  1. Copying and pasting numbers for a bunch of property values
  2. Using preprocessors like Sass with built-in math capabilities
  3. Using the native CSS calc() function

The following examples employ option #3, using PostCSS in order to provide browser support for calc() and var(). If you prefer a different CSS processor, you can apply the same operations with that tool’s own arithmetic and variable syntax.2

For more complex modular scale applications, you should check out these plugins available for PostCSS and Sass:

They provide a cleaner syntax than calc() for getting scale values, and also support multiple values for base and ratio.

Configuring the variables

The first step is to define values for the ratio and base variables that much of the scale logic depends on:

:root {
  --ratio: 1.2;
  --base: 1;
  --base-em: calc(var(--base) * 1em);
  --base-px: calc(var(--base) * 16px);

There are three base-related properties defined here. This is to provide flexibility when different CSS units (or no units) are needed.

Next, you need to establish the range of scale steps you’re likely to use:

:root {
  --ms0: 1;
  --ms1: var(--ratio);                    /* 1.2   */
  --ms2: calc(var(--ratio) * var(--ms1)); /* 1.44  */
  --ms3: calc(var(--ratio) * var(--ms2)); /* 1.728 */
  --ms4: calc(var(--ratio) * var(--ms3)); /* 2.074 */
  --ms5: calc(var(--ratio) * var(--ms4)); /* 2.488 */
  --ms6: calc(var(--ratio) * var(--ms5)); /* 2.986 */
  --ms7: calc(var(--ratio) * var(--ms6)); /* 3.583 */

With these step properties defined, you can now compute them with your base to get values from your scale:

font-size: calc(var(--base-em) * var(--ms1)); /* 1.2em  */
font-size: calc(var(--base-em) * var(--ms2)); /* 1.44em */

Establishing default sizes

The browser default font-size paired with the scale step of 1.44 for line-height feels like an appropriate choice for body copy:

body {
  font-size: calc(var(--base-em) * var(--ms0));
  line-height: calc(var(--base) * var(--ms2));

Headings probably don’t need to consist of six different sizes (as there are other ways to differentiate them). However, if you do wish to incorporate a different size for each, you can use a series of steps from your scale:

h6 { font-size: calc(var(--base-em) / var(--ms1)); }
h5 { font-size: calc(var(--base-em) * var(--ms0)); }
h4 { font-size: calc(var(--base-em) * var(--ms1)); }
h3 { font-size: calc(var(--base-em) * var(--ms2)); }
h2 { font-size: calc(var(--base-em) * var(--ms3)); }
h1 { font-size: calc(var(--base-em) * var(--ms4)); }

In addition to heading font-size, you should also consider the values for line-height.

Using a lower step of the scale in this case can benefit readability when lines are wrapping:

h2 { line-height: calc(var(--base) * var(--ms1)); }
h1 { line-height: calc(var(--base) * var(--ms0)); }

With these defaults for body copy and headings in place, you have a nice foundational hierarchy that works well on small screens:

Ratio of 1.2 on a small screen

A scale with a ratio of 1.2 applied to content on a 320×480 screen

Expanding your scale when proportional contrast is needed

Increasing screen sizes and viewing distances will reveal new typographic requirements for your layout. Your body copy and the contrast in scale of all typographic elements will likely need adjustments to suit the new context:

@media (min-width: 480px) {
  html { font-size: calc(var(--base-px) * var(--ms1)); }

Bumping the global font size like this is a practical adjustment that requires little effort. While this practice is common, it’s worth noting that even small tweaks like this can leverage steps from your modular scale. Rather than incrementing the size by an arbitrary few pixels or a percentage, you can use your scale to determine the new value.

In line with amplifying your body copy, you might also want to increase the amount of change between each heading size. You can do this by overriding the larger headings with sizes from higher up your scale:

@media (min-width: 768px) {
  h3 { font-size: calc(var(--base-em) * var(--ms3)); }
  h2 { font-size: calc(var(--base-em) * var(--ms4)); }
  h1 { font-size: calc(var(--base-em) * var(--ms5)); }
@media (min-width: 1024px) {
  h2 { font-size: calc(var(--base-em) * var(--ms5)); }
  h1 { font-size: calc(var(--base-em) * var(--ms6)); }
@media (min-width: 1360px) {
  h1 { font-size: calc(var(--base-em) * var(--ms7)); }

With these size adjustments in place, viewport width will determine how far up the scale your headings are allowed to climb:

(View the full CodePen to see how the text size responds.)

Broadening the usage of your scale

Limiting your scale’s influence to just specific elements would be highly illogical. You want flexible, abstracted styles to build complex interfaces with. Utility classes can provide that flexibility:

.u-textBigger { 
  font-size: calc(var(--base-em) * var(--ms1)); 
.u-textSmaller { 
  font-size: calc(var(--base-em) / var(--ms1)); 

With one ratio relating all the text sizes, anything affected by these classes will adhere to the steps of your scale. Even when combining or nesting the classes, the resulting text will remain in relative proportion:

<div class="u-textBigger">
  <h2>Now I'm as big as an H1</h2>
  <h3>Now I'm as big as an H2</h3>
  <p class="u-textSmaller">I haven't changed at all.</p>
<div class="u-textSmaller">
  <div class="u-textSmaller">
    <div class="u-textBigger">
      <span class="u-textBigger">Same as it ever was.</span>


  • Use a modular scale to define your typographic proportions.
  • Create a scale that is flexible enough to work on many screen sizes.
  • Use higher steps from your scale when more proportional contrast is needed.
  • Don’t limit the use of your scale to specific elements.

Further reading

  1. Using one base and one ratio isn’t the only way to arrive at a scale that works within small screen limitations. You can also create a multi-stranded scale by using multiple base or ratio values instead of just one. Here is an example using a secondary base value to shorten the distance between each step. See for a better explanation.
  2. The postcss-cssnext plugin can be used to transform the output of calc() and var().

Responsive Images 101, Part 9: Image Breakpoints

I’ve dreaded writing this installment of the Responsive Images 101 series. Selecting image breakpoints is something everyone will face, and frankly, I have no good answers for you.

But sooner or later, we will all face the image breakpoints koan. We might as well start now.

What are responsive image breakpoints?

In our responsive layouts, breakpoints refer to the viewport sizes at which we make changes to the layout or functionality of a page. These typically map to media queries.

Responsive image breakpoints are similar, but slightly different. When I think about image breakpoints, I’m trying to answer two questions:

  • How many image sources do I need to provide to cover the continuum of sizes that this image will be used for?
  • Where and when should the various image sources be used?

The answers to these questions lead to different breakpoints than the criteria we use to select breakpoints for our responsive layouts. For our layouts, we follow Stephen Hay’s advanced methodology: We resize the browser until the page looks bad and then BOOOOM, we need a breakpoint.

With the exception of art direction, the main reason why we need multiple image sources has nothing to do with where the images look bad. We want to provide multiple image sources because of performance concerns, different screen densities, etc.

So we can’t simply reuse our responsive layout breakpoints for our images. Or I guess we can, but if we do so, we’re not really addressing the fundamental reasons why we wanted responsive images in the first place.

Image breakpoints for art direction is relatively easy

In situations where the image falls under the art direction use case, the art direction itself will often tell us how many image sources we need and when they should be used.

If you think back to the Nokia browser site example, we can tell when the image switches from landscape to portrait mode. When that switch occurs, we know we’re going to need a new source image.

However, this may only be part of the picture. What if one of the art directed images covers a large range of sizes. We may find that we still need to have multiple sources that don’t map to the art direction switch.

You can see an example of this in the Shopify homepage that we looked at in Part 8.

Shopify home page animated

Despite the fact that the image only has one major art direction change—from the full image to the cropped one—Shopify still provided six image sources to account for file size and display density.

  <source srcset="homepage-person@desktop.png, homepage-person@desktop-2x.png 2x"       
          media="(min-width: 990px)">
  <source srcset="homepage-person@tablet.png, homepage-person@tablet-2x.png 2x" 
          media="(min-width: 750px)">
  <img srcset="homepage-person@mobile.png, homepage-person@mobile-2x.png 2x" 
       alt="Shopify Merchant, Corrine Anestopoulos">

So knowing that an image falls under the art direction use case can give us some clues, but it doesn’t answer all of our questions about the necessary image breakpoints.

What about resolution switching breakpoints

This is where things really get tricky. At least art direction provides us with some hints about how many image sources might be needed.

So long as we’re downscaling flexible images, they will always look good. We can’t rely on them looking bad to tell us when we need to change image sources.

Let’s take a look at a resolution switching example:

Photo of Michelle Obama at three sizes

In this example, we have a photo of Michelle Obama where the image in the page is 400 x 602 pixels for the current viewport size. The largest size that the image is ever displayed at is 2000 x 3010 pixels. That large file is 250K.

We can simply shrink that 2000-pixel image, and it will look good. But it would be unnecessarily large. It would be better if we provided a smaller version like the one 800 x 1204 resolution image that is shown in the example. That image is only 73K.

We can all agree that when the image in the page is only 400 x 602 pixels in size, that providing an image that is 800×1204 and 73K is better than having people download the largest version of the image.

But why stop at 800×1204?

Michelle Obama example with a fourth size at 600px wide

If we provided another image source that was 600×903 pixels wide, it would only be 42K. That saves us 31K (42%) from the 800×1204 image.

Well shoot. A savings of 42% is a big deal. Maybe we should keep going. 500 pixels wide? 450 pixels wide?

Photo of Michelle Obama with examples at 450 and 500 pixels wide

Each smaller image source offers the potential for substantial savings over the previous size. If we keep on this track, we eventually end up with an image source that is the exact size of the image in the page.

So here’s the question that has vexed me about image breakpoints. How do I know when an image source is too big for the size that the image is being used in the page?

The answer is that unless the image source matches exactly the size that the image is being displayed in the page, it is always going to be too big. There is always going to be an opportunity to optimize it further by providing a smaller image.

Why not provide the exact size of the image?

At this point, you may be wondering why we simply don’t provide the exact size of that the image is going to be used in the page.

First, the whole point of flexible images in responsive design is to provide images that scale as the size of the viewport changes. If we provided images that were exactly the size used in the page, we’d likely need to download new images whenever the viewport size changes or the device was rotated.

Second, it is unrealistic to provide images at any size imaginable. Yes, we can dynamically resize images, but when we resize images, the server needs to do that work which slows down delivery of that image to the browser.

For this reason, most larger sites cache images on content delivery networks (CDN). Caching every image size possible on the CDN would be incredibly expensive.

Finally, the browser doesn’t know the exact size of the image in the page when it starts downloading. That’s what got us to new responsive images standards in the first place!

Possible ways to pick image breakpoints

As I mentioned at the beginning, I have no rock solid solutions for how to pick the number of image sources that you need. Instead, I want to describe some different ways of looking at the problem that may help inform your decisions.

Winging it (aka, matching your layout breakpoints)

Someone on your team says, “Hey, how many source images do you think we need for these product photos?”

You ponder for a moment and say, “Hmm… how about three? Small, medium and large.”

Don’t be ashamed if you’ve done this. I’m pretty sure almost every person working on responsive images has done this at some point.

Perhaps your organization still thinks about mobile, tablet and desktop which makes small, medium and large make sense.

Or maybe you take a look at the range that the image will be displayed and make your best guess. Perhaps you simply look at the number of major layout breakpoints and decide to do the same for your image breakpoints.

I completely understand. And this is better than providing one huge image for all viewports.

But it sure would be nice to have more logic behind our decisions.

Testing representative images

If guessing doesn’t seem like a sound strategy, then let’s insert a little science into the art of picking image breakpoints. We can take a look at some representative images and figure out how many breakpoints they need.

The hardest part of doing this is determining representative images, or figuring out if you have any at all.

For some sites, all the photographs may have a particular style dictated by the brand. If that is the case, finding representative images is easy. Pick a few images and then resize them and save them at sizes ranging between the largest and the smallest images until you feel like you’ve got decent coverage.

Of course, if your site has a diversity of image styles, finding representative images can be nearly impossible.

Memory usage influencing the distribution of image breakpoints

Earlier this summer, Tim Kadlec gave a presentation on Mobile Image Processing. In that presentation, he took a look at the memory usage of flexible images in responsive designs.

What Tim showed is that as an image gets bigger, the impact of resizing an image gets larger.

Memory usage of two different images

In the example above, reducing a 600×600 pixel image by 50 pixels in each direction results in 230,000 wasted bytes versus the 70,000 wasted bytes caused by reducing a 200×200 image by 50 pixels in the exact same way.

Knowing this tells us a bit about how we should pick our breakpoints. Instead of spacing out breakpoints evenly, we should have more breakpoints as the image gets larger.

graph showing more breakpoints at large sizes

Unfortunately, while this tells us that we should have more breakpoints at larger sizes, it doesn’t tell us where those breakpoints should be.

Setting image breakpoints based on a performance budget

What if we applied the idea of a performance budget to responsive images? What would that look like?

We’d start by defining a budget for the amount of wasted bytes that the browser would be allowed to download above what is needed to fit the size of the image in the page.

So say that we decided that we had a performance budget of 20K for each responsive image. That would mean that we would need to make sure that the various sources that we’ve defined for the image are never more than 20K apart.

When we do this, we find that the number of image breakpoints change wildly based on the visual diversity of the image and the compression being used.

Let’s take a look at three sample images.

Time Square — 8 Image Breakpoints

Times Square

This image has a lot of visual diversity. The variations in colors and textures means that JPEG’s lossy compression cannot do as much without damaging the image quality.

Because of this, there are eight image breakpoints—set at 20k intervals—between the smallest size of the image (320×213) and the largest size of the image (990×660).

Breakpoint # Width Height File Size
1 320 213 25K
2 453 302 44K
3 579 386 65K
4 687 458 85K
5 786 524 104K
6 885 590 124K
7 975 650 142K
8 990 660 151K

Morning in Kettering — 3 Image Breakpoints

Morning in Kettering

Unlike the Times Square image, this image has a lot of areas with very similar colors and little variation. Because of this, JPEG can compress the image better.

On an image that can be compressed better, our 20K budget goes farther. For this image, we only need three image breakpoints to cover the full range of sizes that the image will be used at.

Breakpoint # Width Height File Size
1 320 213 9.0K
2 731 487 29K
3 990 660 40K

Microsoft Logo — 1 Image Breakpoint

Microsoft Logo

This is a simple PNG8 file. At its largest size (990×660), it is only 13K. Because of this, it fits into our 20K budget without any modifications.

Breakpoint # Width Height File Size
1 990 660 13K

Take a look at the other images on a sample page we created. See how the number of breakpoints vary even through all the images start with the same resolution end points.

Now, I’m not suggesting that you manually decide on image breakpoints for every single image. But I can envision a future where you might be able to declare to your server that you have a performance budget of 20K for responsive images and then have the server calculate the number of image sources on a per image basis.

I’ve written in more detail about performance budgets for responsive images in the past. If you end up implementing this approach, please let me know.

Setting image breakpoints based on most frequent requests

At a recent Responsive Images Community Group (RICG) meeting, Yoav Weiss and Ilya Grigorik discussed a different way of picking image breakpoints based on the most frequently requested image sizes.

For both Yoav, who works at Akamai, and Ilya, who works at Google, one of the problems they see with multiple image sources is storing all of those sources on edge servers where storage is usually more limited and costs are higher.

Not only do companies like Akamai and Google want to reduce the number of images stored at the edge, but the whole purpose of their content delivery networks is to reduce the amount of time it takes for people to render a web page.

Therefore, if they can cache the most commonly requested image sizes at the edge, they will deliver the fastest experience for the majority of their users.

For these organizations, they can tie their image processing and breakpoints logic to their analytics and change the size of the images over time if they find that new image sizes are getting requested more frequently.

When combined with the new HTTP Client Hints feature that Ilya has championed, servers could get incredibly smart about how to store images in their CDNs and do so in a way that requires little to no decision-making by designers and developers.

Humans shouldn’t be doing this

I believe that in a few years time, no one will be talking about how to pick responsive image breakpoints because no one will be doing it manually.

Sure, we may still make decisions for images that fall into the art direction use case, but even then, we’re probably not going to make finite decisions about every image source. We’ll handle the places that require our intervention and let our image processing services handle the rest.

There is a ton of benefit to either picking image sources based on a performance budget or based on the frequency with which different sizes of the image are requested. But either of these solutions are untenable as part of a manual workflow.

In the future, our typical workflow will be that we upload the highest quality image source into our content management system or image processing system and never have to think about it again.

Part 10 of a 9-part series

This started as a 9-part series, but there is always more to discuss when it comes to responsive images. Read for the conclusion of this series where I’ll provide some essential resources and talk about what the future holds for responsive images.

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
  10. Conclusion

Responsive Images 101, Part 8: CSS Images

Most of the time when people refer to responsive images, they are referring to inline images, not CSS images.

This is because before <picture> and srcset there were no good solutions for inline responsive images. When it comes to CSS images, we could always use media queries. So why worry?

But now it is time to revisit responsive CSS images and look at the solutions anew based on what we’ve learned about inline images.

image-set() for resolution switching

Just like when we’re working with inline images, one of the first questions we’ll need to ask ourselves is whether we’re dealing with the resolution switching or art direction use case.

For resolution switching, we should strive to provide the browser with options and let the browser pick the best possible image. The browser is in a better position to know what image will work best based on user preference, network conditions, etc.

To provide the browser with options, we should use the image-set() syntax.

image-set() syntax repeated below

You may notice some similarity between image-set() and srcset. In fact, srcset was modeled after image-set().

background-image: image-set( "foo.png" 1x, "foo-2x.png" 2x);

Like srcset, image-set’s value contains a comma-separated list of image URIs along with a display density descriptor. If a display density descriptor isn’t provided, it is assumed to be 1x.

However, image-set() does not support width descriptors yet. The plan is to refine image-set() to provide feature parity with srcset.

While most of the examples you will see for image-set() show it applied to background-image, it can be applied to any CSS property that accepts images.

image-set(): The forgotten responsive images standard

image-set() was the first responsive images specific syntax, and as mentioned, it was the foundation for srcset.

However, because we had solutions for CSS responsive images using media queries, image-set() was ignored by nearly everyone. The Responsive Images Community Group didn’t spend much time discussing it. Browsers didn’t prioritize implementing it.

Once we were nearing completion of the <picture> and srcset standards, we looked around and realized that we had neglected image-set(). Work is underway to increase the functionality of image-set() to bring it in line with srcset.

But as of publication, despite being the first responsive images standard, browser support for image-set() is lacking. It is available with a webkit prefix in Chrome, Opera, and Safari. Neither Firefox nor Microsoft have implemented it yet.

So why include it in this Responsive Images 101 series?

Because image-set() is the correct solution for resolution switching. When image-set() is widely supported, we should use it for all of the same reasons we use srcset instead of <picture> with the media attribute when we’re dealing with resolution switching.

Until image-set() is widely supported, you’ll probably end up using the CSS art direction solution for resolution switching.

Art direction

What is the CSS solution for art direction? Media queries.

It’s that simple. In fact, I’m going to assume you know media queries so no syntax sample here.

But as long as I’ve got your ear, make sure your media queries for images don’t overlap or you’ll end up with duplicate downloads. If you have any doubts, check out Tim Kadlec’s Media Query & Asset Downloading Results.

Resolution media queries

If you want to support high density displays in art direction, you’ll probably want to use the new resolution media queries.

Resolution media query syntax. Repeated below.

The resolution media query allows you to apply specific CSS rules to devices that meet the display density that you define.

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
/* High density stuff here */

(Much thanks to CSS Tricks for this syntax sample.)

The first thing you’ll notice in the syntax above is that we’re including a -webkit prefixed media query. This is for devices that support the old device-pixel-ratio syntax. The only devices that got in the wild with this syntax used the -webkit prefix which is why it is the only one listed.

The syntax going forward is the resolution media query. In our example, we’re using min-resolution, but as you probably guessed, there is a complimentary max-resolution feature that can be used instead.

The resolution media query can check one of three things:

  • dpi — dots per inch
  • dpcm — dots per centimeter
  • dppx — dots per px unit

The first two are fairly straight-forward, but I found dppx confusing. The Mozilla Developer Network documentation defines ddpx thusly:

This unit represents the number of dots per px unit. Due to the 1:96 fixed ratio of CSS in to CSS px, 1dppx is equivalent to 96dpi, that corresponds to the default resolution of images displayed in CSS as defined by image-resolution.

Confused? I certainly was when I first read it.

Here’s the way I’ve begun to think about it, the idea of 1x, 2x, 3x, etc. is based on an imprecision. The value of 1x on some devices is different than others because some devices are 72dpi or 96dpi or whatever.

But from a CSS perspective, these differences don’t matter. The CSS Working Group has decided that there will always be a 1:96 fixed ratio of CSS inches to CSS pixels.

So while 1x might leave things up to interpretation because of 72dpi vs. 96dpi screens, 1dppx will always be what you and I think of as “1x”.

You may be asking yourself, why was 1x sufficient for srcset and image-set, but for min-resolution, it was necessary to use dppx?

I don’t know. All I know is that you can think of 1dppx as 1x, 2dppx as 2x, and so on. At this point, I’ve just accepted the inconsistency and decided to move on with my life. I recommend you do the same. ;-)

Now comes the hard part

Believe it or not, responsive images syntax is the easy part. In Part 9, we’ll discuss the vexing challenge of picking your image breakpoints.

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
  10. Conclusion

Responsive Images 101, Part 7: Type

So far we’ve been focused on how to make responsive images more performant. That’s essential, but at the end of the day, we still have the same old image on the page.

Now, it is time for the fun stuff!

Type attribute

Have you ever bemoaned the fact that your options for reliable image formats are limited to jpg, png, and gif? Ever wanted found yourself wondering if there is enough browser support for new formats like SVG or webp?

If so, you’re going to love the type attribute.

Type syntax repeated below

The type attribute can be added to <source> elements inside a <picture> element and allows you to declare different image types that the browser can choose from:

  <source type="image/svg+xml" srcset="logo.xml">
  <source type="image/webp" srcset="logo.webp"> 
  <img src="logo.png" alt="ACME Corp">

This new type attribute is modeled on the <video> element’s type attribute and works the same way.

The browser will pick the first source where the declared image type is one that it supports. If it doesn’t recognize any of the source types, it will use the <img> element’s src or srcset declarations.

The value is a MIME type for the image format being referenced in the srcset attribute. If you have multiple image URIs listed in the srcset attribute, they should all match the declared image MIME type.

Of course, you can combine type with the sizes and/or the media attributes as well. All three of these attributes are optional and can be combined to accomplish whatever you need.

The srcset attribute is required for all <source> elements. Both display density and width descriptors can be used with the type attribute.

Do you need the media attribute?

I’ve gotten in the habit of telling people that they shouldn’t use the <picture> element for most responsive images. That is both true and a bit misleading.

So now that you’re up to speed on all of the inline responsive images techniques, let’s break it down:

  • Most images on the web fit the resolution switching use case.
  • When you’ve got a resolution switching use case, you want to empower the browser to make the best choice possible. This is what srcset is designed to do.
  • When you use the <picture> element with media attributes, you’re dictating to the browser what images it should use.

Therefore, you can and should use <picture> when you want both resolution switching and to support multiple image formats. Just leave off the media attribute so that the browser can do its thing.

Progressive enhancement for image formats

So far in this series, I’ve tried to keep things professional, but lighthearted. But that ends here because…


Phew, had to get that out of my system.

For years we’ve wanted to be able to use different image formats, but had to wait for wide spread adoption of the format.

But even when we finally felt we could switch, we always knew that we were ignoring old browsers. We’d chalk it up to progress and hope it didn’t affect too many people. Or maybe we never switched to the new image formats for fear of leaving people out.

But <picture> plus the type attribute gives use a way out of this conundrum. We can use progressive enhancement for image formats right now.

Sara Soueidan described how she is starting to do this for SVG with PNG fallbacks instead of all the hacks we used to use.

But it’s not just SVG and webp. What about JPEG-2000? JPEG-XR? APNG?

If you can find browsers that support an image format and you believe it can provide some value to your users, then there is no reason not to use that format so long as you provide alternatives.

JPEG-2000? Yes please!

A wonderfully in-depth article by Zoltan Hawryluk opened my eyes to the benefits of different image formats and in particular JPEG-2000 for alpha transparency images.

In one of the examples in Zoltan’s article, he shows dice placed above a multi-color background. To pull it off requires an alpha-channel transparency.


The file sizes of the dice image are:

320×240 2K 22.6K 55.2K 112.1K
600×450 13.5K 48.5K 14.3K 26.6K
1024×768 19.2K 95.7K 325.7K 56K

Look at those savings. The dice PNG at 1024×768 is 325.7K. The same image as JPEG-2000 is only 19.2K. That’s insane!

I know what you’re thinking. That’s wonderful, but no browsers support JPEG-2000.

That’s what I thought too, but I was wrong. Both desktop and Mobile Safari already support JPEG-2000.

Now before you go converting all of your images to JPEG-2000, heed Zoltan’s warning:

As you can see, the numbers for JPEG-2000 are especially impressive. However, the file sizes of the alternate images will vary depending on the characteristics of the original image… While alternative image formats may give better results, sometimes they don’t.

So it will depend on the image and the design. But you can see how there can be significant benefits for some of your users depending on the types of images and the formats their browsers support.

Brave new world of image formats

I don’t expect anyone to go off and immediately start using JPEG-2000. There’s a lot more work to be done in this space so that we know what image formats make sense and when to use them.

Simply getting tools in place to output the various images formats can be difficult. Zoltan includes information at the bottom of his article on what tools he used to create the different formats.

Other than the command-line tools, I find the tools to be awkward and rough around the edges. There has been no incentive for companies like Adobe to add rich support for image formats like JPEG-2000 because no one could use them until now.

We have a lot of experimenting to do. I can’t wait!

What about CSS?

Everything we’ve talked about so far has been for inline responsive images. Because we already had media queries in CSS, inline responsive images were the biggest challenge and most of the focus has been on them.

But there are some new standards for responsive images in CSS and a few tricks you should know. Continue on for Part 8: CSS Responsive Images.

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
  10. Conclusion