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For the last 2 years, the Production Leadership Team has been closely monitoring the progress of the Bootstrap project in regards to its next major release, Bootstrap 5. We do have a big interest in shipping Joomla with the latest and greatest versions of the libraries and frameworks we use. Because of the problems which arose after the release of Joomla 3 and Bootstrap 2, we were very torn on how to proceed on this front.
Switching the major version of a framework like this is not an easy decision and we want share the arguments which we have had to consider:

Especially in the last 2 meetings of the PLT, we have had intense discussions about this and initially we leaned towards not switching the version. However the work of Dimitris Grammatiko informed our opinion. It turned out that changing the version could be done faster and easier than expected. Inspired by that PR, George Wilson tested this out for the CSS as well and with him making fast progress, we are now leaning towards doing this switch here and now.

Above we listed several questions, so let us also list the arguments for each question as well:

Joomla 4 already has many great features to ship with.
Developers will have the benefit of a next generation CSS framework decoupled from jQuery and packed with the features they need to make their extensions perform at their best. It will make integration with other frameworks easier.
Joomla 4 will also benefit from Right to Left support, all reasons to embrace this change.
The upgrading to Bootstrap 5 will finish off the work of building a new major Joomla version with an up to date CSS framework and ensure the longevity of the J4 release well into the future.

We wish all well, stay safe and healthy.

Production Department