Migrating to Bootstrap 5.2

An experience report in migrating Docsy from Bootstrap 4 to 5.2, with insights and instructions.

Docsy, and Docsy-based project websites (including those at the CNCF), have been happily using the Bootstrap CSS framework from Docsy’s inception. In January of this year, Bootstrap 4 (the version used by Docsy for the past few years) reached its end of life. While we, the Docsy steering committee, have been eager to benefit from the Bootstrap 5 improvements, we were concerned about the magnitude of the migration effort, as well as the impact on downstream projects. Because of this, the migration was delayed for as long as possible. In December of 2022, when Bootstrap 4 stopped receiving critical upgrades, we declared Docsy to be in a feature freeze, and focused our maintenance efforts on the Bootstrap 5 migration.

This post is about Docsy’s migration journey to Bootstrap 5.21: it highlights the most notable steps, with a special attention given to the most surprising aspects of the migration. Our hope is that this post will be useful to others upgrading to Bootstrap 5, in particular, for downstream Docsy projects — though we plan a separate post specifically for downstream projects.


Eager to dive into the Bootstrap migration of your project? Besides carefully stepping through the Bootstrap migration page, watch out for the following:

  • The media-breakpoint-down() mixin’s breakpoint argument needs to be shifted.
  • Grid .row and .col style changes are breaking.
  • Import ordering of Bootstrap Sass files: import functions first.

For details, read on.

Technical details

If you are well accustomed to upgrading Docsy (and its dependencies) by reading changelogs and systematically stepping through commits, then this section provides a summary of some notable changes. In it, I describe technical aspects of the migration that surprised me, either because they required particular care in fixing, were undocumented, and/or insufficiently explained in the Bootstrap migration page.

Mixin media-breakpoint-down() argument shift

The breakpoint argument to the media-breakpoint-down() mixin needs to be bumped up to the next higher breakpoint. Thankfully, a similar change isn’t required of media-breakpoint-up(). This change will be required of Docsy-based projects. If you forget to make this non-obviously breaking layout change, your project’s responsive layouts will likely start misbehaving in apparently strange ways.

For details and an example, see:

Grid .row and .col style changes are breaking

The main issue addressed in this section is not, at the time of writing, documented in the Bootstrap 5 migration page.

There seems to be an assumption, in Bootstrap 5, that the immediate child of a .row should be a .col. I don’t know how strict an assumption this is. While I have searched for an explicit statement of this assumption in the Bootstrap documentation, I haven’t found one yet — if you are aware of such a statement, let us know!

This assumption wasn’t apparent nor was it enforced in Bootstrap 4, consequently, some of Docsy’s layouts failed to respect it. In most cases, fixing violations consisted of simply wrapping a .row’s child element in a .col, but the Docsy footer required a couple of iterations to get right.

My first footer adjustment reset flex-shrink to its default value (PR #1373), but that turned out to be unnecessary once I better understood how to appropriately handle row margins (PR #1523) — rows have negative margins, as I recently learned, which is something to keep in mind.

The following Bootstrap 5 .col style changes influenced Docsy-specific style updates and might impact Docsy-based projects as well:


Import ordering of Bootstrap Sass files: functions first

Projects can import Bootstrap Sass sources all in one go (using bootstrap.scss), or selectively import any one of the 40+ Bootstrap parts, layouts, and components that they need. Regardless of the import strategy chosen, due to a Sass map initialization limitation, Bootstrap-client projects need to perform (emphasis mine):

… variable customizations … after @import "functions", but before > @import "variables" and the rest of [the Bootstrap] import stack.

For details, see New _maps.scss from the migration page, and Importing from Bootstrap’s Sass customization documentation.

Having to maintain a custom list of a few dozen imports (even if it’s relatively stable) feels like a maintenance overhead that we should avoid if we can, so in Docsy’s main.scss, we @import “functions” before Docsy- and project-specific variable overrides, and then we import the full Bootstrap suite of SCSS. This results in _functions.scss being imported twice, but according to the Sass @import documentation:

If the same stylesheet is imported more than once, it will be evaluated again each time. If it just defines functions and mixins, this usually isn’t a big deal, but if it contains style rules they’ll be compiled to CSS more than once.

The _functions.scss file only contains function definitions, so we should be ok. This seems like a small cost to pay in contrast to the alternative strategy of inlining the 40+ imports from bootstrap.scss.


Systematic and stepwise migration

If you’ve glanced at the Bootstrap 5 migration page, you will see that there are a lot of changes to address while migrating. To ensure that we didn’t miss any, we systematically walked through the migration guide, and tracked the status of each change through Docsy issue #470. Each relevant migration page section is represented in the issue’s opening comment: we either noted that a migration-page section is irrelevant for Docsy, or added the section to the tracking issue, and list the PRs containing corresponding Docsy-specific changes. If you’re curious to see how that worked out, see Upgrade to Bootstrap 5.2 · Docsy issue #470.

First Bootstrap 5 release of Docsy

A first Bootstrap 5 release of Docsy is planned for the start of June, since most aspects of the migration have been completed. Some updates have been postponed, most notably support for right-to-left (RTL) text. For the complete list of followup items, see BSv5.2 upgrade followup · Docsy issue #1510.

As was mentioned earlier, this first release will be in support of Bootstrap 5.2. We plan a separate migration effort to bring Docsy up to Bootstrap 5.3, in particular to benefit from new color modes. You can track our progress through Docsy issue #1528.

Migrating Docsy-based projects

This section contains some preliminary and general guidance for downstream projects. We are planning a separate post to cover more migration details.

Bootstrap migration-page walkthrough

Each project uses its own specific set of Bootstrap features, so walking through the Bootstrap 5.2 migration page will be advisable for most projects. Of course, one strategy is just to upgrade and see what breaks or no longer works, but only doing that without a more systematic follow-up would be ill-advised for all but the most trivial projects—consider the challenge in detecting and recovering from a missed change to a ​​media-breakpoint-down() argument, as discussed earlier.

Docsy-specific changes

During the migration effort we seized the opportunity to do some long overdue Docsy house cleaning. For details concerning both breaking and non-breaking Docsy-specific changes, consult the changelog. In particular, one non-breaking but important change to be aware of is: [BSv5] Docsy variables cleanup … PR #1462.

Give it a try!

To get a first and quick impression of the impact of the upgrade on your project, it can be informative to simply upgrade Docsy and see what breaks. This is what the Docsy team did with Bootstrap 5. Only one change actually broke the build of the Docsy User Guide: the rename of the color-yiq() function.

After such a smoke test, we recommend systematically walking through the Bootstrap migration page as described above, and the Docsy changelog. I used this approach for opentelemetry.io, which was the first Docsy-based project to be upgraded with a pre-release of Bootstrap-5-based Docsy. The upgrade went quite smoothly. The main pain point of the OTel website was upgrading to Bootstrap 5 forms; an aspect of the migration that didn’t apply to Docsy since Docsy uses only the most trivial of forms.

We’ll have more to share about the OTel migration effort as well as general project-specific migration advice in a followup blog post. In the meantime, I hope that you have found parts of this technical article helpful for your own migration efforts.

CNCF project websites eager to migrate can send questions to the CNCF #techdocs Slack channel. CNCF and other Docsy-based projects can also start a discussion in the Docsy repository. Happy migrating!

A big thanks to the Docsy Steering Committee and other reviewers who offered feedback on earlier drafts of this post, as well as to all those who contributed to the migration effort.

A version of this article originally appeared as the CNCF blog post Migrating Docsy to Bootstrap 5 .

  1. Bootstrap 5.3 reached GA on May 30. There will be a separate migration effort to bring Docsy up to Bootstrap 5.3↩︎