Add custom CSS preprocessor

What’s in this topic

This topic describes how to add a custom CSS preprocessor. Adding Sass support is used as an example.

Sample Sass module

Magento has a sample module-sample-scss module implementing the Sass preprocessor.

You can view it as example when adding your custom preprocessor. Or install the module as is if you need to add Sass preprocessing. Installing a module is described in the repository’s Readme file.

Adding a custom preprocessor


For the sake of compatibility, upgradability and easy maintenance, do not edit the default Magento code. Create a new custom module for your customizations.

For details about creating a module refer to the Magento PHP Developer Guide.

Step by step instruction

To add a custom preprocessor, take the following steps:

  1. In your module directory, add the adapter PHP class. It must implement the Magento\Framework\View\Asset\ContentProcessorInterface interface. For illustration, see the adapter for Sass in the sample module: module-sample-scss/Preprocessor/Adapter/Scss/Processor.php

  2. If the browser compilation is possible for your file types, that is, if the corresponding JavaScript library exists, create the custom renderer for the client-side compilation. This will allow the default client-side compilation functionality to be applied for your files type as well. You can use the default Magento renderer for reference: app/code/Magento/Developer/Model/View/Page/Config/ClientSideLessCompilation/Renderer.php

  3. If in your custom preprocessor, the syntax of the importing directives is different from @import and @magento_import, you must implement custom processor classes. You can view the default Magento processors for reference: lib/internal/Magento/Framework/Css/PreProcessor/Instruction.

  4. In <your_module_dir>/etc/di.xml, declare the following:

    • your custom adapter
    • your processor (if relevant)
    • the renderer for the client-side compilation (if relevant)

The content of your di.xml will be similar to the following:


<?xml version="1.0"?>
<config xmlns:xsi="" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <virtualType name="AlternativeSourceProcessors">
            <argument name="alternatives" xsi:type="array">
                <item name="%your_preprocessor_name%" xsi:type="array">
                    <item name="class" xsi:type="string">%Your\Adapter\Class%</item>
                <!-- Use the following syntax to set the priority of processors. That is, what file types will the system search for, when requested CSS files are not found. The following lines set SCSS to be prior to Less -->
                <item name="less" xsi:type="array">
                    <item name="after" xsi:type="string">scss</item>
    <!-- Add the following declaration if you have custom processors for importing directives -->
    <virtualType name="AssetPreProcessorPoolForSourceThemeDeploy" type="Magento\Framework\View\Asset\PreProcessor\Pool">
            <argument name="preprocessors" xsi:type="array">
                <item name="%your_preprocessor%" xsi:type="array">
                    <item name="magento_import" xsi:type="array">
                        <item name="class" xsi:type="string">%Your\Import\Processor%</item>
                    <item name="import" xsi:type="array">
                        <item name="after" xsi:type="string">magento_import</item>
                        <item name="class" xsi:type="string">%Your\Magento_import\Processor%</item>
    <!-- Declare the renderer for client-side compilation -->
<type name="Magento\Developer\Model\View\Page\Config\RendererFactory">
            <argument name="rendererTypes" xsi:type="array">
                <item name="client_side_compilation" xsi:type="string">%Your\Client\Side\Renderer%</item>