Zulip CSS organization

There are two high-level sections of CSS: the “portico” (logged-out pages like /help/, /login/, etc.), and the app. The Zulip application’s CSS can be found in the web/styles/ directory, while the portico CSS lives under the web/styles/portico/ subdirectory.

To generate its CSS files, Zulip uses PostCSS and a number of PostCSS plugins, including postcss-nesting, whose rules are derived from the CSS Nesting specification.

Editing Zulip CSS

If you aren’t experienced with doing web development and want to make CSS changes, we recommend reading the excellent Chrome developer tools guide to the Elements panel and CSS, as well as the section on viewing and editing CSS to learn about all the great tools that you can use to modify and test changes to CSS interactively in-browser (without even having the reload the page!).

Our CSS is formatted with Prettier. You can ask Prettier to reformat all code via our linter tool with tools/lint --only=prettier --fix. You can also integrate it with your editor.

Zulip’s development environment has hot code-reloading configured, so changes made in source files will immediately take effect in open browser windows, either by live-updating the CSS or reloading the browser window (following backend changes).

CSS style guidelines

Avoid duplicated code

Without care, it’s easy for a web application to end up with thousands of lines of duplicated CSS code, which can make it very difficult to understand the current styling or modify it. We would very much like to avoid such a fate. So please make an effort to reuse existing styling, clean up now-unused CSS, etc., to keep things maintainable.

Opt to write CSS in CSS files. Avoid using the style= attribute in HTML except for styles that are set dynamically. For example, we set the colors for specific streams ({{stream_color}}) on different elements dynamically, in files like user_stream_list_item.hbs:

  class="stream-privacy-original-color-{{stream_id}} stream-privacy filter-icon"
  style="color: {{stream_color}}">

But for most other cases, its preferable to define logical classes and put your styles in external CSS files such as zulip.css or a more specific CSS file, if one exists. See the contents of the web/styles/ directory.

Be consistent with existing similar UI

Ideally, do this by reusing existing CSS declarations, so that any improvements we make to the styling can improve all similar UI elements.

Use clear, unique names for classes and object IDs

This makes it much easier to read the code and use git grep to find where a particular class is used.

Don’t use the tag name in a selector unless you have to. In other words, use .foo instead of We shouldn’t have to care if the tag type changes in the future.

Additionally, multi-word class and ID values should be hyphenated, also known as kebab case. In HTML, opt for class="my-multiword-class", with its corresponding CSS selector as .my-multiword-class.

Validating CSS

When changing any part of the Zulip CSS, it’s important to check that the new CSS looks good at a wide range of screen widths, from very wide screen (e.g. 1920px) all the way down to narrow phone screens (e.g. 480px).

For complex changes, it’s definitely worth testing in a few different browsers to make sure things look the same.

HTML templates


  • Templates are automatically recompiled in development when the file is saved; a refresh of the page should be enough to display the latest version. You might need to do a hard refresh, as some browsers cache webpages.

  • Variables can be used in templates. The variables available to the template are called the context. Passing the context to the HTML template sets the values of those variables to the value they were given in the context. The sections below contain specifics on how the context is defined and where it can be found.

Backend templates

For text generated in the backend, including logged-out (“portico”) pages and the web app’s base content, we use the Jinja2 template engine (files in templates/zerver).

The syntax for using conditionals and other common structures can be found here.

The context for Jinja2 templates is assembled from a couple places:

  • zulip_default_context in zerver/ This is the default context available to all Jinja2 templates.

  • As an argument in the render call in the relevant function that renders the template. For example, if you want to find the context passed to index.html, you can do:

$ git grep zerver/app/index.html '*.py'
zerver/views/    response = render(request, 'zerver/app/index.html',

The next line in the code being the context definition.

Frontend templates

For text generated in the frontend, live-rendering HTML from JavaScript for things like the main message feed, we use the Handlebars template engine (files in web/templates/) and sometimes work directly from JavaScript code (though as a policy matter, we try to avoid generating HTML directly in JavaScript wherever possible).

The syntax for using conditionals and other common structures can be found here.

There’s no equivalent of zulip_default_context for the Handlebars templates.


Handlebars is in our package.json and thus ends up in node_modules; We use handlebars-loader to load and compile templates during the webpack bundling stage. In the development environment, webpack will trigger a browser reload whenever a template is changed.


All user-facing strings (excluding pages only visible to sysadmins or developers) should be tagged for translation.


Zulip uses TippyJS for its tooltips.

Static asset pipeline

This section documents additional information that may be useful when developing new features for Zulip that require front-end changes, especially those that involve adding new files. For a more general overview, see the new feature tutorial.

Our dependencies documentation has useful relevant background as well.

Primary build process

Zulip’s frontend is primarily JavaScript in the web/src directory; we are working on migrating these to TypeScript modules. Stylesheets are written in CSS extended by various PostCSS plugins; they are converted from plain CSS, and we have yet to take full advantage of the features PostCSS offers. We use Webpack to transpile and build JS and CSS bundles that the browser can understand, one for each entry points specified in web/webpack.*assets.json; source maps are generated in the process for better debugging experience.

In development mode, bundles are built and served on the fly using webpack-dev-server with live reloading. In production mode (and when creating a release tarball using tools/build-release-tarball), the tools/update-prod-static tool (called by both tools/build-release-tarball and tools/upgrade-zulip-from-git) is responsible for orchestrating the webpack build, JS minification and a host of other steps for getting the assets ready for deployment.

You can trace which source files are included in which HTML templates by comparing the entrypoint variables in the HTML templates under templates/ with the bundles declared in web/webpack.*assets.json.

Adding static files

To add a static file to the app (JavaScript, TypeScript, CSS, images, etc), first add it to the appropriate place under static/.

  • Third-party packages from the NPM repository should be added to package.json for management by pnpm, this allows them to be upgraded easily and not bloat our codebase. Run ./tools/provision for pnpm to install the new packages and update its lock file. You should also update PROVISION_VERSION in in the same commit.

  • Third-party files that we have patched should all go in web/third/. Tag the commit with “[third]” when adding or modifying a third-party package. Our goal is to the extent possible to eliminate patched third-party code from the project.

  • Our own JavaScript and TypeScript files live under web/src. Ideally, new modules should be written in TypeScript (details on this policy below).

  • CSS files live under web/styles.

  • Portico JavaScript (“portico” means for logged-out pages) lives under web/src/portico.

  • Custom SVG graphics living under web/images/icons are compiled into custom icon webfonts by webfont-loader according to the web/images/icons/template.hbs template.

For your asset to be included in a development/production bundle, it needs to be accessible from one of the entry points defined either in web/webpack.assets.json or web/

  • If you plan to only use the file within the app proper, and not on the login page or other standalone pages, put it in the app bundle by importing it in web/src/bundles/app.ts.

  • If it needs to be available both in the app and all logged-out/portico pages, import it to web/src/bundles/common.ts which itself is imported to the app and common bundles.

  • If it’s just used on a single standalone page which is only used in a development environment (e.g. /devlogin) create a new entry point in web/ or it’s used in both production and development (e.g. /stats) create a new entry point in web/webpack.assets.json. Use the bundle macro (defined in templates/zerver/base.html) in the relevant Jinja2 template to inject the compiled JS and CSS.

If you want to test minified files in development, look for the DEBUG = line in zproject/ and set it to False.

How it works in production

A few useful notes are:

  • Zulip installs static assets in production in /home/zulip/prod-static. When a new version is deployed, before the server is restarted, files are copied into that directory.

  • We use the VFL (versioned file layout) strategy, where each file in the codebase (e.g. favicon.ico) gets a new name (e.g. favicon.c55d45ae8c58.ico) that contains a hash in it. Each deployment, has a manifest file (e.g. /home/zulip/deployments/current/staticfiles.json) that maps codebase filenames to serving filenames for that deployment. The benefit of this VFL approach is that all the static files for past deployments can coexist, which in turn eliminates most classes of race condition bugs where browser windows opened just before a deployment can’t find their static assets. It also is necessary for any incremental rollout strategy where different clients get different versions of the site.

  • Some paths for files (e.g. emoji) are stored in the rendered_content of past messages, and thus cannot be removed without breaking the rendering of old messages (or doing a mass-rerender of old messages).

ES6/TypeScript modules

JavaScript modules in the frontend are ES6 modules that are transpiled by webpack. Any variable, function, etc. can be made public by adding the export keyword, and consumed from another module using the import statement.

New modules should ideally be written in TypeScript (though in cases where one is moving code from an existing JavaScript module, the new commit should just move the code, not translate it to TypeScript). TypeScript provides more accurate information to development tools, allowing for better refactoring, auto-completion and static analysis. TypeScript also uses the ES6 module system. See our documentation on TypeScript static types.

Webpack does not ordinarily allow modules to be accessed directly from the browser console, but for debugging convenience, we have a custom webpack plugin (web/debug-require-webpack-plugin.ts) that exposes a version of the require() function to the development environment browser console for this purpose. For example, you can access our people module by evaluating people = require("./src/people"), or the third-party lodash module with _ = require("lodash"). This mechanism is not a stable API and should not be used for any purpose other than interactive debugging.

We have one module, zulip_test, that’s exposed as a global variable using expose-loader for direct use in Puppeteer tests and in the production browser console. If you need to access a variable or function in those scenarios, add it to zulip_test. This is also not a stable API.