README

This module customises Silverstripe's requirements backend to defer CSS loading using the preload pattern.

It overrides includeInHTML() and rewrites stylesheet links to:

<link rel="preload" as="style" href="..." onload="this.onload=null;this.rel='stylesheet'">

This helps reduce initial render-blocking caused by CSS files while still applying styles once each file has loaded.

What this module changes

Replaces the default SilverStripe\View\Requirements_Backend via Injector.
Overrides includeInHTML() in Toast\Injectors\ToastEnhancedBackend.
Keeps JavaScript and custom head tags injection behavior aligned with the parent backend flow.
Converts CSS requirements to rel="preload" links with an onload switch to rel="stylesheet".

Configured in _config/config.yml:

SilverStripe\Core\Injector\Injector:
  SilverStripe\View\Requirements_Backend:
    class: Toast\Injectors\ToastEnhancedBackend

composer require toastnz/deferred-css

Then flush configuration:

vendor/bin/sake dev/build flush=all

In ToastEnhancedBackend::includeInHTML():

Checks if </head> exists and there are requirements to inject.
Runs processCombinedFiles() to preserve combined assets behavior.
Outputs JavaScript includes and inline custom scripts.
Rewrites CSS includes as preload links with as="style" and onload rel swap.
Injects CSS/head tags into <head> and scripts according to Silverstripe settings.

This module targets Silverstripe Framework 6.
onload-based deferred CSS is widely used, but always test critical rendering paths in your project.
Validate frontend behavior in older browsers used by your audience.

BSD-3-Clause. See LICENSE.