We support current standards and best practices of displaying images in modern web browsers. Although we will always be working to expand the methods of image display we support, the sheer breadth of variables prevents us from being compatible with every method.

Supported image formats

✅ We support testing on responsive <picture> elements with <source> and <img> children with srcset attributes.

Example:

<picture>
    <source srcset="https://chartbeat.com/large.jpg, https://chartbeat.com/large-retina.jpg 2x" media="(min-width: 1200px)">
    <source srcset="https://chartbeat.com/medium.jpg, https://chartbeat.com/medium-retina.jpg 2x" media="(min-width: 976px)">
    <source srcset="https://chartbeat.com/small.jpg, https://chartbeat.com/small-retina.jpg 2x" media="(min-width: 576px)">
    <img srcset="https://chartbeat.com/mobile.jpg" alt="A picture">
</picture>

✅ We support testing on responsive <img> elements with src and srcset attributes.

Example:

<img src="https://chartbeat.com/image.jpg" srcset="https://chartbeat.com/image1x.jpg 1x, https://chartbeat.com/image2x.jpg 2x">

✅ We support testing on non-responsive <img> elements with a single src attribute defined on page load.

Example:

<img src="https://chartbeat.com/image.jpg">

✅ We support testing on images with a single lazy load src or srcset attribute for any of the above formats (i.e. data-src or any other attribute name that your website uses). We do not support testing on homepages that use multiple unique lazy load attribute identifiers, such as data-image-small, data-image-medium, data-image-large.

Example:

<img data-src="https://chartbeat.com/image.jpg">

Unsupported image formats

❌ We do not support testing for <img> elements that do not belong to an <a> tag (anchor element). Tests can only be initiated on <img> elements that belong to an anchor tag linking to the target article.

Example:

<!-- Unsupported -->
<article> 
  <img src="image.jpg">
  <a href="test.com/article1">
    <h3>Headline Text</h3>
  <a>
</article>
​
<!-- Supported -->
<article> 
  <a href="test.com/article1">
    <img src="image.jpg">
  </a>
  <a href="test.com/article1">
    <h3>Headline Text</h3>
  <a>
</article>
​
<!-- Also supported -->
<article> 
  <a href="test.com/article1">
    <img src="image.jpg">
    <h3>Headline Text</h3>
  <a>
</article>

❌ We do not support testing for <picture> tags that do not properly have an <img> element as a child.

Example:

<picture><div class="background-css-image-here"></div></picture>

❌ We do not support testing images that use the background-image CSS property.

Example:

<div style="background-image: url('image.jpg');"></div>

❌ We do not support testing on responsive <img> elements that have their src attributes dynamically altered by JavaScript.

• This can usually be determined by inspecting an <img> element at mobile and desktop breakpoints: if the srcs differ at each breakpoint, JavaScript is dynamically altering the src.

• Chartbeat code does not have a way to determine whether this is or is not the case as the markup is indistinguishable from supported elements. In the rare case that your site uses this as well as supported formats, use caution when making a test.

❌ We do not support testing on <picture> or <img> elements that are injected dynamically into the page by JavaScript dependent on user action.

Partial homepage compatibility

If your site uses both supported and unsupported formats for different positions on your homepage, you will only be able to successfully run image tests on supported formats. You will still be able to run headline tests on positions with unsupported image formats. Inspect your page layouts to confirm if you can run image tests in each section.

Next steps

Read through the next article to learn about headline and image flicker, and if necessary, how to prevent it with our optional implementation to prevent flicker, called "flicker control".