CSS Debugging and Optimization: Code-quality Tools


The following introduction to CSS code-quality tools is an extract from Tiffany’s upcoming book, CSS Master, 2nd Edition, which will be available shortly.

On your road to becoming a CSS master, you’ll need to know how to troubleshoot and optimize your CSS. How do you diagnose and fix rendering problems? How do you ensure that your CSS creates no performance lags for end users? And how do you ensure code quality?

Knowing which tools to use will help you ensure that your front end works well.

In this article, we’ll discuss tools that help you analyze the quality of your CSS. We’ll focus on two:

  • stylelint
  • UnCSS

stylelint is a linting tool. A linter is an application that checks code for potential trouble spots, enforcing coding conventions such as spaces instead of tabs for indentation. stylelint can find problems such as duplicate selectors, invalid rules, or unnecessary specificity. These have the greatest impact on CSS maintainability.

UnCSS, on the other hand, checks your CSS for unused selectors and style rules. It parses a stylesheet and a list of HTML pages, returning a CSS file that’s stripped of unused rules.

Both of these tools use Node.js and can be installed using npm.

If you’re working on a small site, such as a personal blog or a few pages that are updated infrequently, many of the problems that these tools flag can safely be ignored. You’ll spend time refactoring for little gain in maintainability and speed. For larger projects, however, they’re invaluable. They’ll help you head off maintainability problems before they start.

stylelint

stylelint helps you avoid errors and enforce conventions in your styles. It has more than 160 error-catching rules and allows you to create your own as well via plugins.

stylelint Installation and Configuration

Install stylelint as you would any other npm package:

npm install -g stylelint

Once it’s installed, we’ll need to configure stylelint before using it. stylelint doesn’t ship with a default configuration file. Instead, we need to create one. Create a .stylelistrc file in your project directory. This file will contain our configuration rules, which can use JSON (JavaScript Object Notation) or YAML (YAML Ain’t Markup Language) syntax. Examples in this section use JSON.

Our .stylelistrc file must contain an object that has a rules property. The value of rules will itself be an object containing a set of stylelist rules and their values:

{
    "rules": {}
}

If, for example, we wanted to banish !important from declarations, we can set the declaration-no-important to true:

{
    "rules": {
        "declaration-no-important": true
    }
}

stylelint supports over 150 rules that check for syntax errors, indentation and line-break consistency, invalid rules, and selector specificity. You’ll find a complete list of rules and their available values in the stylelint User Guide.

Starting with a Base stylelint Configuration

You’ll probably find it easier to start with a base configuration and then customize it to your project needs. The stylelint-config-recommended base configuration is a good starting configuration. It enables all of the “possible errors” rules. Install it using npm:

npm install -g stylelint-config-recommended

Then, in your project directory, create a .stylelistrc file that contains the following lines:

{
    "extends": "/absolute/path/to/stylelint-config-recommended"
}

Replace /absolute/path/to/ with the directory to which stylelint-config-recommended was installed. Global npm packages can usually be found in the %AppData%npmnode_modules directory on Windows 10 systems, and in /usr/local/lib/node_modules on Unix/Linux and macOS systems. Type npm list -g to locate your global node_modules directory.

We can then augment our configuration by adding a rules property. For example, to disallow vendor prefixes, our .stylelistrc file would look similar to the what’s below:

{
    "extends": "/absolute/path/to/stylelint-config-recommended",
    "rules": {
       "value-no-vendor-prefix": true
    }
}

What if we wanted to limit the maximum specificity of our selectors to 0,2,0? That would permit selectors such as .sidebar .title but not #footer_nav. We can do this by adding a selector-max-specificity rule to our configuration:

{
    "extends": "/absolute/path/to/stylelint-config-recommended",
    "rules": {
       "value-no-vendor-prefix": true,
       "selector-max-specificity": "0,2,0"
    }
}

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