WebP, an image format developed by Google, which is intended to replace JPEG, PNG, and GIF file formats, will soon be generated by default for new JPEG image uploads in WordPress and used for website content. The main work for this feature was committed to core for inclusion in the upcoming WordPress 6.1 release.

The initial proposal was revised after significant critical feedback. The most notable changes include automatically generating WebP versions of only core image sizes, keeping secondary (WebP) sub-sizes only if they are smaller than the primary MIME type, and only generating WebP images for image sizes that are intended for use in user-facing front-end content.

Despite a raft of revisions, and filters to control or disable WebP uploads, the proposal remained controversial. Contributors continue to report issues after testing. Many still have reservations about whether this should be opt-in or on by default.

“When converting medium-resolution photographs (approx 1600px – 2500px on the long edge), WebP files are often larger than the JPEG equivalent,” WordPress developer Mark Howells-Mead commented on the main ticket for WebP work. “(In my tests using my own photography, in around 60% of cases.) This change might make the ‘modern image format’ test of Page Speed Insights happy, but enforcing WebP by default on sites which use a lot of photography will often cause longer image loading times.”

Some developers are supportive of the change but prefer for it to be off by default when it is first rolled out, to allow the ecosystem to prepare for the change.

“I definitely see it as a big advantage to add Core support for additional MIME types for sub-sized image files,” Matthias Reinholz said. “But I can’t see adding conversion to a specific other file format as preferred behavior. This may help to optimize the market position of WebP but it will also be a serious threat to plugin authors and existing larger websites that do not pay attention to this change.

“Therefore, I’m questioning why this functionality should be activated by default at this stage. IMHO, it should be opt-in only. Plus ideally, we would already start to think about adding further image formats to be supported by this feature.”

NerdPress founder Andrew Wilder created a separate ticket urging contributors to consider making the feature opt-in, but the ticket was closed and conversation directed back to the main ticket so as not to splinter the discussion.

“Making these new features opt-in instead of opt-out would be the best way to be cautious about potential impacts,” Wilder said.

“There have been many requests for this to be opt-in (as well as some asking for a setting on the Media page, rather than only a filter for developers). So far there hasn’t been any open conversation about why that’s not being taken into consideration.”

The notion that WebP by default should be opt-in was summarily dismissed and the conversation was not revisited before the changes were committed.

“The feature will have widespread benefits for users by opting in core sizes (to start) – if it were entirely opt-in it would have little impact – or benefit,” Google-sponsored Core Committer Adam Silverstein said in response to opponents.

In response to suggestions that this feature ship with a UI for enabling it on the media page, Silverstein said, “We have discussed both suggestions in chats and issues with mixed responses. Project philosophy is regularly mentioned as aligning with the current approach.”

The ticket remains open awaiting patches for a few loose threads on the technical implementation. Contributors have continued to chime in with additional concerns.

The Performance team has a new blog where people can follow updates on their current projects and proposals. Now that the main WebP work has been committed, the next steps will discussed in future meetings with notes posted to the new Core Performance blog.