require-css

Optimizable CSS requiring with RequireJS

For LESS inclusion, use require-less, which behaves and builds the css exactly like this module apart from the preprocessing step.

Overview

Allows the construction of scripts that can require CSS, using the simple RequireJS syntax:

define(['css!styles/main'], function() {
  //code that requires the stylesheet: styles/main.css
});

CSS Requiring

• CSS requiring CSS is downloaded and injected into the page. Url path normalization of assets within the CSS file is managed.
• Fully compatible load callback The plugin call is loaded once the CSS is downloaded and injected. This method works across all browsers and devices. It bypasses the onload support issues that make this a tricky problem otherwise (http://requirejs.org/docs/faq-advanced.html#css).
• Cross-domain CSS Cross-domain CSS is loaded with the use of a <link> tag, but build and onload support are not provided for these - the callback fires instantly.

CSS Building

• CSS builds When run as part of a build with the RequireJS optimizer, css! dependencies are automatically inlined into the built layer within the JavaScript, fully compatible with layering. CSS injection is performed as soon as the layer is loaded.
• Option to build separate layer CSS files A separateCSS build parameter allows for built layers to output their css files separately, instead of inline with the JavaScript, for manual inclusion.
• CSS compression CSS redundancy compression is supported through the external library, csso.

Installation and Setup

Download the require-css folder manually or use volo(npm install volo -g):

volo add guybedford/require-css

To allow the direct css! usage, add the following map configuration in RequireJS:

map: {
  '*': {
    'css': 'require-css/css'
  }
}

Use Cases and Benefits

Motivation

The use case for RequireCSS came out of a need to manage templates and their CSS together. The idea being that a CSS require can be a dependency of the code that dynamically renders a template. When writing a large dynamic application, with templates being rendered on the client-side, it can be beneficial to inject the CSS as templates are required instead of dumping all the CSS together separately. The added benefit of this is then being able to build the CSS naturally with the RequireJS optimizer, which also supports separate build layers as needed.

Script-inlined CSS Benefits

By default, during the build CSS is compressed and inlined as a string within the layer that injects the CSS when run.

If the layer is included as a <script> tag, only one browser request is needed instead of many separate CSS requests with <link> tags.

Even better than including a layer as a <script> tag is to include the layer dynamically with a non-blocking require. Then the page can be displayed while the layer is still loading asynchronously in the background. In this case, the CSS that goes with a template being dynamically rendered is loaded with that same script asynchronously. No longer does it need to sit in a <link> tag that blocks the page display unnecessarily.

Injection methods

Previous attempts have used the onLoad callback for a <link> tag to register CSS require completion. This method is limited since while it is supported in all versions of IE, it has varying support in other browsers and mobile devices. Because it can't reliably work, there are many hacks which are used to get around this.

In a typical use case, one doesn't mind if the assets have completed downloading yet. Hence the main CSS load requirement is that the CSS has been downloaded and parsed.

CSS parsing speeds are quick enough that a dynamic injection will in most cases be instantanous. Thus, there is no real disadvantage to the method used here.

CSS content is downloaded as text, injected into a <style> tag, and the load callback is run immediately after injection. Typically this would be followed by a rendering stage, and this hasn't resulted in any content flashes whatsoever in tests so far across devices.

Note: It is advisable to avoid import tags in CSS files, as the injection callback will not be able to detect when these are loaded.

If CSS resources such as images are important to be loaded first, these can be added to the require through a loader plugin that can act as a preloader such as image or font. Then a require can be written of the form:

require(['css!my-css', 'image!preload-background-image.jpg', 'font!google,families:[Tangerine]']);

Modular CSS

RequireCSS implies a CSS modularisation where styles can be scoped directly to the render code that they are bundled with.

Just like JS requires, the order of CSS injection can't be guaranteed. The idea here is that whenever there are style overrides, they should be based on using a more specific selector with an extra id or class at the base, and not assuming a CSS load order. Reset and global styles are a repeated dependency of all modular styles that build on top of them.

Development Environment

When developing on a local file server, an AJAX request will be made to load each CSS file. This can conflict with browser origin settings on the local server as AJAX requests are by default not allowed to other files.

To enable this, configure the browser to allow this, or set CORS headers.

As an example, some configuration help for Chrome is given here.

Optimizer Configuration

Basic Usage

Optimizer configuration:

{
  modules: [
  {
    name: 'mymodule'
  }
  ]
}

If the contents of 'mymodule' are:

  define(['css!style', 'css!page'], function(css) {
    //...
  });

Then the optimizer output would be:

-mymodule.js containing: style.css and page.css which will be dynamically injected

Separate File Output

To output the CSS to a separate file, use the configuration:

{
  separateCSS: true,
  modules: [
  {
    name: 'mymodule'
  }
  ]
}

This will then output all the css to the file mymodule.css. This configuration can also be placed on the module object itself for layer-specific settings.

Optimization is fully compatible with exclude and include.

Note: Optimization will only work when using r.js version 2.1.0 or later (released Oct 4 2012)

CSS Compression

CSS compression is supported with csso.

To enable the CSS compression, install csso with npm:

  npm install csso -g

The build log will display the compression results.

When running the r.js optimizer through NodeJS, sometimes the global module isn't found. In this case install csso as a local node module so it can be found.

Conditional CSS

Some styles are conditional on the environment. For example mobile stylesheets and IE-specific stylesheets.

To manage this, use the Require-IS module.

With Require-IS, one can do:

require(['is!mobile?css!mobile-css'], function(css) {
  //...
});

Mobile detection can be defined through a detection script in Require-IS, such as:

mobile.js:

define(function() {
  return navigator.userAgent.match(/iPhone/); //(just iphone detection as an example)
});

Separate build layers can then be made for mobile specific use. Read more at the Require-IS project page.

Roadmap

• Comprehensive CSS minification including style reduction
• LESS extension
• Sprite compilation
• Source maps?

Suggestions always appreciated - feel free to post a feature request.