HTML::DeferableCSS - Simplify management of stylesheets in your HTML


version v0.4.2


  use HTML::DeferableCSS;

  my $css = HTML::DeferableCSS->new(
      css_root      => '/var/www/css',
      url_base_path => '/css',
      inline_max    => 512,
      simple  => 1,
      aliases => {
        reset => 1,
        jqui  => 'jquery-ui',
        site  => 'style',
      cdn => {
        jqui  => '//',

  $css->check or die "Something is wrong";


  print $css->deferred_link_html( qw[ jqui site ] );


This is an experimental module for generating HTML-snippets for deferable stylesheets.

This allows the stylesheets to be loaded asynchronously, allowing the page to be rendered faster.

Ideally, this would be a simple matter of changing stylesheet links to something like

  <link rel="preload" as="stylesheet" href="....">

but this is not well supported by all web browsers. So a web page needs to use some JavaScript to handle this, as well as a noscript block as a fallback.

This module allows you to simplify the management of stylesheets for a web application, from development to production by

  • declaring all stylesheets used by your web application;

  • specifying remote aliases for stylesheets, e.g. from a CDN;

  • enable or disable the use of minified stylesheets;

  • switch between local copies of stylesheets or CDN versions;

  • automatically inline small stylesheets;

  • use deferred-loading stylesheets, which requires embedding JavaScript code as a workaround for web browsers that do not support these natively.



This is a required hash reference of names and their relative filenames to "css_root".

It is recommended that the .css and .min.css suffixes be omitted.

If the name is the same as the filename (without the extension) than you can simply use 1. (Likewise, an empty string or 0 disables the alias:

  my $css = HTML::DeferableCSS->new(
    aliases => {
        reset => 1,
        gone  => 0,       # "gone" will be silently ignored
        one   => "1.css", #

If all names are the same as their filenames, then an array reference can be used:

  my $css = HTML::DeferableCSS->new(
    aliases => [ qw( foo bar } ],

If an alias is disabled, then it will simply be ignored, e.g.


Returns an empty string. This allows you to disable a stylesheet in your configuration without having to remove all references to it.

Absolute paths cannot be used.

You may specify URLs instead of files, but this is not recommended, except for cases when the files are not available locally.


This is the required root directory where all stylesheets can be found.


This is the URL prefix for stylesheets.

It can be a full URL prefix.


If true (default), then a file with the .min.css suffix will be preferred, if it exists in the same directory.

Note that this does not do any minification. You will need separate tools for that.


This is a hash reference used internally to translate "aliases" into the actual files or URLs.

If files cannot be found, then it will throw an error. (See "check").

This is a hash reference of "aliases" to URLs. (Only one URL per alias is supported.)

When "use_cdn_links" is true, then these URLs will be used instead of local versions.

This is true when there are "cdn_links".

When true, this will prefer CDN URLs instead of local files.


This specifies the maximum size of an file to inline.

Local files under the size will be inlined using the "link_or_inline_html" or "deferred_link_html" methods.

Setting this to 0 disables the use of inline links, unless "inline_html" is called explicitly.


True by default.

This is used by "deferred_link_html" to determine whether to emit code for deferred stylesheets.


When true, a noscript element will be included with non-deffered links.

This defaults to the same value as "defer_css".


This is the pathname of the cssrelpreload.js file that will be embedded in the resulting code.

The script comes from

You do not need to modify this unless you want to use a different script from the one included with this module.

This is a code reference for a subroutine that returns a stylesheet link.


This is a code reference for a subroutine that returns a stylesheet preload link.


This is an optional static asset id to append to local links. It may refer to a version number or commit-id, for example.

This is useful to ensure that changes to stylesheets are picked up by web browsers that would otherwise use cached copies of older versions of files.


True if there is an "asset_id".


This is a code reference for logging errors and warnings:

  $css->log->( $level => $message );

By default, this is a wrapper around Carp that dies when the level is "error", and emits a warning for everything else.

You can override this so that errors are treated as warnings,

  log => sub { warn $_[1] },

or that warnings are fatal,

  log => sub { die $_[1] },

or even integrate this with your own logging system:

  log => sub { $logger->log(@_) },


When true, this enables a simpler method of using deferable CSS, without the need for the loadCSS script.

It is false by default, for backwards compatability. But it is recommended that you set this to true.


This attribute was added in v0.4.0.



This method instantiates lazy attributes and performs some minimal checks on the data. (This should be called instead of "css_files".)

It will throw an error or return false (depending on "log") if there is something wrong.

This was added in v0.3.0.


  my $href = $css->href( $alias );

This returns this URL for an alias.

  my $html = $css->link_html( $alias );

This returns the link HTML markup for the stylesheet referred to by $alias.


  my $html = $css->inline_html( $alias );

This returns an embedded stylesheet referred to by $alias.

  my $html = $css->link_or_inline_html( @aliases );

This returns either the link HTML markup, or the embedded stylesheet, if the file size is not greater than "inline_max".

Note that a stylesheet will be inlined, even if there is are "cdn_links".

  my $html = $css->deferred_link_html( @aliases );

This returns the HTML markup for the stylesheets specified by "aliases", as appropriate for each stylesheet.

If the stylesheets are not greater than "inline_max", then it will embed them. Otherwise it will return the appropriate markup, depending on "defer_css".


Content-Security-Policy (CSP)

If a web site configures a Content-Security-Policy setting to disable all inlined JavaScript, then the JavaScript shim will not work.

XHTML Support

This module is written for HTML5.

It does not support XHTML self-closing elements or embedding styles and scripts in CDATA sections.


All files are embedded as raw files.

No URL encoding is done on the HTML links or "asset_id".

It's spelled "Deferrable"

It's also spelled "Deferable".


The development version is on github at and may be cloned from git://


Please report any bugs or feature requests on the bugtracker website

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

Please report any bugs in cssrelpreload.js to


Robert Rothenberg <>

This module was developed from work for Science Photo Library

reset.css comes from

cssrelpreload.js comes from


This software is Copyright (c) 2020 by Robert Rothenberg.

This is free software, licensed under:

  The MIT (X11) License