Install, test and build Readium CSS

[Implementers’ doc]

Install and init references for regression tests

First, navigatate to the project’s folder in your terminal,

cd path/to/readium-css

then type:

npm install

This will install all dev dependencies needed and make npm scripts available to ease all processes you’ll need to run later.

Then, once the install is finished, type:

npm run test:ref

This will create reference screenshots for the CSS regression tests.


We are using PostCSS, a tool for transforming CSS with JavaScript. It comes with a vast amount of task-oriented plugins and allows authors to use modern specs which are not implemented yet.

PostCSS Dependencies

ReadiumCSS is relying on a PostCSS config to build dist stylesheets. If you npm install the repository, all those dependencies will be installed as well.

Here is the current list of dependencies:

Build dist stylesheets

If you customize ReadiumCSS-config.css, you will have to rebuild stylesheets.

Note: the current build process is subpar – to say the least. Please feel free to improve it (gulp, grunt, etc.).

Available scripts

By default, the following scripts are available:

Those scripts will overwrite the files in the css/dist folder, the stylesheets you’ll use in your app.


First navigate to the readium-css folder if you didn’t already, then…

npm run build

Building dist stylesheets for browsers which don’t support CSS variables

If you need to build stylesheets for IE11 or an early version of Edge (e.g. 14), then you can use most of ReadiumCSS, excepted user settings. You’ll consequently have to customize the src’s ReadiumCSS-before.css, ReadiumCSS-default.css and ReadiumCSS-after.css and remove the user settings submodules.

Then you must customize the selectors in ReadiumCSS-config.js and replace them with either CSS classes or custom attributes so that reading modes and flags can work as expected.

Finally you will have to enable the postcss-css-variables and postcss-alter-property-value in the postcss.config.js file to be found at the src folder’s root.

The following must be added to plugins:

  "preserve": true
  declarations: {
    "*": {
      task: "remove"
    , whenValueEquals: "undefined"

This will:

  1. interpolate CSS variables into a static representation, while preserving variables for other browsers ("preserve": true);
  2. remove static representations which can’t be interpolated and are undefined (remove task).

We recommend managing user settings via JavaScript in this case, especially as you can test support for CSS variables, as described in the CSS Variables primer.

Useful PostCSS plugins

Here is a list of additionnal PostCSS plugins which might prove useful to implementers.


Once you have build dist stylesheets, you can run regression tests using Backstop.js.

It helps you check if you didn’t accidentally create a breaking change when customizing stylesheets, and make sure pagination, reading modes, and user settings work as expected.


You will find the configuration file, backstop.json at the root of the project. By default, it runs those tests for a smartphone (portrait) and a tablet (landscape) viewports using Chrome, but you can customize it to fit your needs.

For instance, if you don’t need to support mobile, you could modify viewports:

"viewports": [
    "label": "desktop small",
    "width": 800,
    "height": 600
    "label": "desktop large",
    "width": 1600,
    "height": 900

And if you want to run tests using Webkit instead of Blink because you’re developing iOS apps:

"engine": "phantomjs"

Test files

If you customize flags in ReadiumCSS-config.css, you must modify HTML files in the tests folder; user settings are indeed set as inline styles on the html element and are using the default flags.

Available scripts

By default, the following scripts are available:


First navigate to the readium-css folder if you didn’t already, then…

npm run test

The regression tests will run against the newly-created dist stylesheets, which is why you must build them beforehand.

Once all scenarios are tested for the viewports you created, which can take up to a minute, a report will automatically open in your browser.

If a unit test is marked as “failed”, it doesn’t necessarily mean the user setting failed, it just means you made a significant change which impacts rendering. Take a closer look at the diff, and if you’re happy with the result, head to the terminal and type:

npm run test:approve

This will make the current test screenshots the new reference for the next test.

Note: on some occasions, an error might happen during tests and the process won’t stop. Try ctrl + c to stop the current process and run the test again.