Delegating CSS utility classes generation to Tailwind CSS

Summary

Styling elements in GitLab primarily relies on CSS utility classes. Those are classes that generally define a single CSS property and that can be applied additively to change an element's look. We have developed our own tooling in the GitLab UI project to generate the utils we need, but our approach has demonstrated a number of flaws that can be circumvented by delegating that task to the Tailwind CSS framework.

This initiative requires that we deprecate existing utilities so that Tailwind CSS can replace them.

Motivation

In June 2019, we have consolidated our usage of CSS utility classes through RFC#4 which introduced the concept of silent classes, where utilities would be generated from a collection of manually defined SCSS mixins.

This has served us well, but came with some caveats:

• Increased development overhead: whenever a new utility is needed, it has to be manually added to the GitLab UI project. One then needs to wait on a new version of @gitlab/ui to be released and installed in the consumer project.
• Inconsistencies: Without any tooling in place to check how utilities are named, we have seen many inconsistencies make their way in the library, making it quite unpredictable. The most striking example of this was the introduction of desktop-first utilities among a majority of mobile-first utils, without any way of distinguishing the former from the latter, other than looking at the source.
• Disconnection between the utilities library and its consumers: When a utility is added to the library, it is made available to any project that uses @gitlab/ui. As a result, some utils are included in projects that don't need them. Conversely, if all consumers stop using a given util, it could potentially be removed to decrease the CSS bundle size, but we have no visibility over this.
• Limited autocompletion: Although it's possible to configure autocomplete for the existing library, it is restricted to the utilities bundle. In contrast, Tailwind CSS autocomplete aligns with an on-demand approach, ensuring that all utilities are readily available. Additionally, IDE extensions can enhance understanding by revealing the values applied by a specific utility.

As part of this architectural change, we are alleviating these issues by dropping our custom built solution for generating CSS utils, and delegating this task to Tailwind CSS.

It is worth noting that this was previously debated in RFC#107. The RFC was well received. The few concerns that were raised were about the CSS utility approach as a whole, not the way we implemented it. This initiative's purpose is not to question our reliance on utility classes, but to consolidate its implementation to improve engineers' efficiency when working with CSS utils.

Why Tailwind CSS?

Here are a few reasons that led us to choosing Tailwind CSS over similar tools:

• It is a long-standing project that has been battle-tested in many production apps and has a healthy community around it.
• Tailwind CSS is well maintained and keeps evolving without getting bloated.
• It integrates well in all of our tech stacks
  • Ruby on Rails projects can leverage the tailwindcss-rails Gem.
  • Nuxt apps can set up the tailwindcss module.
  • More generic frontend stacks can use the tailwindcss Node module.

Goals

This blueprint's goal is to improve the developer experience (DX) when working with CSS utility classes. As a result of this initiative, frontend engineers' efficiency should be increased thanks to a much lower development overhead.

Non-Goals

As stated in the motivations above, this focuses on improving an existing architectural decision, not on replacing it with a new design. So this therefore:

• Is not aimed at revisiting the way we write CSS or how we apply styles within our projects.
• Does not focus on user-facing improvements. This change is mostly a developer experience enhancement. The resulting increase in efficiency could certainly indirectly improve user experience, but that is not our primary intent.

Proposal

We will be setting up Tailwind CSS in GitLab UI and GitLab. The intent is to have the main Tailwind CSS configuration in GitLab UI. This step is where we'll be maintaining the Pajamas-compliant configuration properties (color, spacing scale, etc.). The Tailwind CSS setup in GitLab will inherit from GitLab UI's setup. The subtlety here is that, in GitLab, we will be scanning both the GitLab codebase and the @gitlab/ui Node module. This will ensure that GitLab UI does not need to expose any CSS utilities anymore, but the ones it relies on are still generated in GitLab. A similar setup will need to be introduced in other projects that use CSS utilities and need to upgrade to the Tailwind CSS-based version.

Pros

• We are removing the cumbersome workflow for adding new utilities. One should be able to use any utility right away without contributing to another project and waiting through the release cycle.
• We are introducing a predictable library, where the naming is decided upon in the overarching Tailwind CSS project. As engineers know, naming things is difficult, and it's best that we defer this to a well-established project.
• Engineers should be able to refer to Tailwind CSS documentation to know what utils are available and how to use them. No need to read through GitLab UI's source code anymore.
• Because Tailwind CSS generates the required utils by scanning the consumer's codebase, we'll be sure to only generate the utilities we actually need, keeping CSS bundle sizes under control. This must be taken with a grain of salt though: Tailwind CSS is extremely flexible and makes it possible to generate all sorts of utils, sometimes with developer-defined values, which could result in large utils bundles depending on how we'll adopt Tailwind CSS' features.
• We'll benefit from a robust IDE integration providing auto-completion and previews for the utils we support.

Cons

• More setup: each project that requires CSS utils would need to have Tailwind CSS set up, which might be more or less tedious depending on the environment.
• One more dev dependency in each project.
• Inability to use string interpolation to build class names dynamically (Tailwind CSS needs to see the full names to generate the required classes).
• We'll need a migration: we'll need to ensure usages of the existing CSS utilities library don't break, which implies a deprecation/migration process.

Design and implementation details

In order to prevent breakages, we are taking an iterative approach to moving away from the current library. The proposed path here is purposefully rough around the edges. We acknowledge that it's not a one size fits all solution and that we might need to adjust to some cases along the way.

Here's the basic process:

1. Deprecate a collection of utility mixins in GitLab UI. This entails replacing the gl- prefix with gl-deprecated- in the mixin's name, and updating all usages in both GitLab UI and GitLab accordingly. We will typically focus on a single mixins file at a time, though we might want to deprecate several files at once if they are small enough. Conversely, some files might be too big to be deprecated in one go and would require several iterations.
2. Enable the corresponding Tailwind CSS core plugins so that we can immediately start using the newer utilities.
3. Migrate deprecated utilities to their Tailwind CSS equivalents.

flowchart TD
    RequiresDeprecation(Is the mixins collection widely used in GitLab?)
    DeprecateMixins[Mark the mixins as deprecated with the `gl-deprecated-` prefix]

    HasTailwindEq(Does Tailwind CSS have equivalents?)
    EnableCorePlugin["Enable the corresponding Tailwind CSS core plugin(s)"]
    WriteCustomUtil[Write a custom Tailwind CSS utility]

    MigrateUtils[Migrate legacy utils to Tailwind CSS]

    RequiresDeprecation -- Yes --> DeprecateMixins
    DeprecateMixins --> HasTailwindEq
    RequiresDeprecation -- No --> HasTailwindEq
    HasTailwindEq -- Yes --> EnableCorePlugin
    HasTailwindEq -- No --> WriteCustomUtil
    EnableCorePlugin --> MigrateUtils
    WriteCustomUtil --> MigrateUtils

The deprecation step gives us some margin to evaluate each migration without risking breakages in production. It does have some drawbacks:

• We might cause merge conflicts for others as we will be touching several areas of the product in our deprecation MRs. We will make sure to communicate these changes efficiently to not make things too confusing. We will also use our best judgement to split MRs when we feel like their scope gets too large.
• Deprecation MRs might require approval from several departments, which is another reason to be transparent and iterative throughout the process.
• We are purposefully introducing technical debt which we are committed to pay in a reasonable time frame. We acknowledge that the actual duration of this initiative may be affected by a number of factors (uncovering edge-cases, DRIs' capacity, department-wide involvement, etc.), but we expect to have it completed in 6-12 months.