Learnings for adopting the standardized loading attribute
My goal with this post is to persuade CMS platform developers and contributors (i.e. the people who develop CMS cores) that now is the time to implement support for the browser-level image lazy loading feature. I'll also share recommendations on how to ensure high-quality user experiences and enable customization by other developers while implementing lazy loading. These guidelines come from our experience adding support to WordPress as well as helping Joomla, Drupal, and TYPO3 implement the feature.
Regardless of whether you're a CMS platform developer or a CMS user (i.e. a person who builds websites with a CMS), you can use this post to learn more about the benefits of browser-level lazy loading in your CMS. Check out the Next steps section for suggestions on how you can encourage your CMS platform to implement lazy loading.
Over the past year,
lazy loading images and iframes using the
become part of the WHATWG HTML Standard
seen growing adoption by various browsers.
These milestones however only lay the groundwork for a
faster and more resource-saving web.
It is now on the distributed web ecosystem to make use of the
Content management systems
power about 60% of websites,
so these platforms play a vital role in bringing adoption of modern browser
features to the web. With a few popular open-source CMSs such as
TYPO3 having already
implemented support for the
loading attribute on images, let's have a look at
their approaches and the takeaways which are relevant for adopting the feature
in other CMS platforms as well. Lazy loading media is a key web performance
feature that sites should benefit from at a large scale, which is why adopting
it at the CMS core level is recommended.
The case for implementing lazy loading now
Adoption of non-standardized browser features in CMSs facilitates widespread testing and can surface potential areas of improvement. However, the general consensus across CMSs is that, as long as a browser feature is not standardized, it should preferably be implemented in the form of an extension or plugin for the respective platform. Only once standardized can a feature be considered for adoption in the platform core.
Browser support of the feature is a similar concern: The majority of CMS users should be able to benefit from the feature. If there is a considerable percentage of browsers where the feature is not yet supported, the feature has to ensure that it at least has no adverse effect for those.
User experience recommendations
Require dimension attributes on elements
In order to avoid layout shifts, it has been a
long-standing recommendation that
embedded content such as images or iframes should always include the dimension attributes
so that the browser can infer the aspect ratio of those elements before actually
loading them. This recommendation is relevant regardless of whether an element
is being lazy-loaded or not. However, due to the
0.1% greater likelihood of an image not being fully loaded once in the viewport
it becomes slightly more applicable with lazy loading in place.
CMSs should preferably provide dimension attributes on all images and iframes. If this is not possible for every such element, they are recommended to skip lazy loading images which do not provide both of these attributes.
Avoid lazy loading above-the-fold elements
At the moment CMSs are recommended to only add
loading="lazy" attributes to
images and iframes which are positioned below-the-fold, to avoid a delay
in the Largest Contentful Paint metric, which in some cases can be
significant as discovered in July 2021. However, it has
to be acknowledged that it's complex to assess the position of an element
relative to the viewport before the rendering process. This applies especially
if the CMS uses an automated approach for adding
loading attributes, but even
based on manual intervention several factors such as the different viewport
sizes and aspect ratios have to be considered. Still, it is strongly recommended to omit hero images and other images or iframes that are likely to appear above the fold from being lazy-loaded.
provide lazy loading to browsers which do not (yet) support the
such mechanisms always rely on initially removing the
src attribute of an
image or iframe, which causes a delay for the browsers that do support the
frontends of a large-scale CMS increases the surface area for potential issues,
which is part of why no major CMS had adopted lazy loading in its core prior to
the standardized browser feature.
Enable lazy loading by default
The overall recommendation for CMSs implementing browser-level lazy loading is to
enable it by default, i.e.
loading="lazy" should be added to images and
only for those elements that include dimension attributes.
Having the feature enabled by default will result in greater network resource
savings than if it had to be enabled manually, for example on a per-image
As much as possible,
only be added to elements which likely appear below-the-fold.
While this requirement can be complex to implement for a CMS due to lack of client-side awareness and various viewport sizes, it is recommended to at least use approximate heuristics to omit elements such as hero images that will likely appear above-the-fold from being lazy-loaded.
Allow per-element modifications
loading="lazy" should be added to images and iframes by default, it is
crucial to allow omitting the attribute on certain images, for example to
optimize for LCP. If the audience of the CMS is on
average considered more tech-savvy, this could be a UI control exposed for every
image and iframe allowing to opt out of lazy loading for that element.
Alternatively or in addition, an API could be exposed to third-party developers
so that they can make similar changes through code.
Retrofit existing content
At a high level, there are two approaches for adding the
loading attribute to
HTML elements in a CMS:
- Either add the attribute from within the content editor in the backend, persistently saving it in the database.
- Add the attribute on the fly when rendering content from the database in the frontend.
It is recommended for CMS to opt for adding the attribute on the fly when rendering, in order to bring the lazy loading benefits to any existing content as well. If the attribute could solely be added through the editor, only new or recently modified pieces of content would receive the benefits, drastically reducing the CMS's impact on saving network resources. Furthermore, adding the attribute on the fly will easily allow for future modifications, should the capabilities of browser-level lazy loading be further expanded.
Adding the attribute on the fly should cater for a potentially existing
loading attribute on an element though and let such an attribute take
precedence. This way, the CMS or an extension for it could also implement the
editor-driven approach without causing a conflict with duplicate attributes.
Optimize server-side performance
When adding the
loading attribute to content on the fly using (for example) a
server-side middleware, speed is a consideration. Depending on the CMS, the
attribute could be added either via DOM traversal or regular expressions, with
the latter being recommended for performance.
Regular expressions use should be kept to a minimum, for example a single regex
which collects all
iframe tags in the content including their
attributes and then adds the
loading attribute to each tag string as
applicable. WordPress for example goes as far as
having a single general regular expression to perform various on-the-fly operations to certain elements,
of which adding
loading="lazy" is just one, using a single regular expression
to facilitate multiple features. This form of optimization furthermore is
another reason why adopting lazy loading in a CMS's core is recommended over an
extension - it allows for better server-side performance optimization.
See if there is an existing feature request ticket to add support for the feature in your CMS, or open a new one if there is none yet. Use references to this post as needed to support your proposal.
Tweet me (felixarntz@) for questions or comments, or to get your CMS listed on this page if support for browser-level lazy loading has been added. If you encounter other challenges, I am also curious to learn more about them to hopefully find a solution.
If you're a CMS platform developer, study how other CMSs have implemented lazy loading:
You can use the learnings from your research and the technical recommendations from this post to start contributing code to your CMS, for example in form of a patch or pull-request.