package HTML::DeferableCSS;

# ABSTRACT: Simplify management of stylesheets in your HTML

use v5.10;
use Moo 1.006000;

use Carp;
use Devel::StrictMode;
use File::ShareDir 1.112 qw/ dist_file /;
use MooX::TypeTiny;
use List::Util 1.45 qw/ first uniqstr /;
use Path::Tiny;
use Types::Path::Tiny qw/ Dir File Path /;
use Types::Common::Numeric qw/ PositiveOrZeroInt /;
use Types::Common::String qw/ NonEmptySimpleStr SimpleStr /;
use Types::Standard qw/ Bool CodeRef HashRef Maybe Tuple /;


use namespace::autoclean;

our $VERSION = 'v0.4.2';

has aliases => (
    is       => 'ro',
    isa      => STRICT ? HashRef [Maybe[SimpleStr]] : HashRef,
    required => 1,
    coerce   => sub {
        return { map { $_ => $_ } @{$_[0]} } if ref $_[0] eq 'ARRAY';
        return $_[0];

has css_root => (
    is       => 'ro',
    isa      => Dir,
    coerce   => 1,
    required => 1,

has url_base_path => (
    is      => 'ro',
    isa     => SimpleStr,
    default => '/',

has prefer_min => (
    is      => 'ro',
    isa     => Bool,
    default => 1,

has css_files => (
    is  => 'lazy',
    isa => STRICT
             ? HashRef [ Tuple [ Maybe[Path], Maybe[NonEmptySimpleStr], PositiveOrZeroInt ] ]
             : HashRef,
    builder => 1,
    coerce  => 1,

use constant PATH => 0;
use constant NAME => 1;
use constant SIZE => 2;

sub _build_css_files {
    my ($self) = @_;

    my $root = $self->css_root;
    my $min  = $self->prefer_min;

    my %files;
    for my $name (keys %{ $self->aliases }) {
        my $base  = $self->aliases->{$name};
        if (!$base) {
            $files{$name} = [ undef, undef, 0 ];
        elsif ($base =~ m{^(\w+:)?//}) {
            $files{$name} = [ undef, $base, 0 ];
        else {
            $base = $name if $base eq '1';
            $base =~ s/(?:\.min)?\.css$//;
            my @bases = $min
                ? ( "${base}.min.css",  "${base}.css", $base )
                : ( "${base}.css",  "${base}.min.css", $base );

            my $file = first { $_->exists } map { path( $root, $_ ) } @bases;
            unless ($file) {
                $self->log->( error => "alias '$name' refers to a non-existent file" );
            # PATH NAME SIZE
            $files{$name} = [ $file, $file->relative($root)->stringify, $file->stat->size ];

    return \%files;

has cdn_links => (
    is        => 'ro',
    isa       => STRICT ? HashRef [NonEmptySimpleStr] : HashRef,
    predicate => 1,

has use_cdn_links => (
    is      => 'lazy',
    isa     => Bool,
    builder => 'has_cdn_links',

has inline_max => (
    is      => 'ro',
    isa     => PositiveOrZeroInt,
    default => 1024,

has defer_css => (
    is      => 'ro',
    isa     => Bool,
    default => 1,

has include_noscript => (
    is      => 'lazy',
    isa     => Bool,
    builder => 'defer_css',

has preload_script => (
    is      => 'lazy',
    isa     => File,
    coerce  => 1,
    builder => sub { dist_file( qw/ HTML-DeferableCSS cssrelpreload.min.js / ) },

has link_template => (
    is      => 'ro',
    isa     => CodeRef,
    builder => sub {
        return sub { sprintf('<link rel="stylesheet" href="%s">', @_) },

has preload_template => (
    is      => 'lazy',
    isa     => CodeRef,
    builder => sub {
        my ($self) = @_;
        if ($self->simple) {
            return sub {
                sprintf('<link rel="preload" as="style" href="%s">' .
                        '<link rel="stylesheet" href="%s" media="print" onload="\'all\';this.onload=null;">',
                        $_[0], $_[0])
        else {
            return sub { sprintf('<link rel="preload" as="style" href="%s" onload="this.onload=null;this.rel=\'stylesheet\'">', $_[0]) };

has asset_id => (
    is        => 'ro',
    isa       => NonEmptySimpleStr,
    predicate => 1,

has log => (
    is => 'ro',
    isa => CodeRef,
    builder => sub {
        return sub {
            my ($level, $message) = @_;
            croak $message if ($level eq 'error');
            carp $message;

has simple => (
    is      => 'ro',
    isa     => Bool,
    default => 0,

sub check {
    my ($self) = @_;

    my $files = $self->css_files;

    scalar(keys %$files) or
        return $self->log->( error => "no aliases" );

    return 1;

sub href {
    my ($self, $name, $file) = @_;
    $file //= $self->_get_file($name) or return;
    if (defined $file->[PATH]) {
        my $href = $self->url_base_path . $file->[NAME];
        $href .= '?' . $self->asset_id if $self->has_asset_id;
        if ($self->use_cdn_links && $self->has_cdn_links) {
            return $self->cdn_links->{$name} // $href;
        return $href;
    else {
        return $file->[NAME];

sub link_html {
    my ( $self, $name, $file ) = @_;
    if (my $href = $self->href( $name, $file )) {
        return $self->link_template->($href);
    else {
        return "";

sub inline_html {
    my ( $self, $name, $file ) = @_;
    $file //= $self->_get_file($name) or return;
    if (my $path = $file->[PATH]) {
        if ($file->[SIZE]) {
            return "<style>" . $file->[PATH]->slurp_raw . "</style>";
        $self->log->( warning => "empty file '$path'" );
        return "";
    else {
        $self->log->( error => "'$name' refers to a URI" );

sub link_or_inline_html {
    my ($self, @names ) = @_;
    my $buffer = "";
    foreach my $name (uniqstr @names) {
        my $file = $self->_get_file($name) or next;
        if ( $file->[PATH] && ($file->[SIZE] <= $self->inline_max)) {
            $buffer .= $self->inline_html($name, $file);
        else {
            $buffer .= $self->link_html($name, $file);
    return $buffer;

sub deferred_link_html {
    my ($self, @names) = @_;
    my $buffer = "";
    my @deferred;
    for my $name (uniqstr @names) {
        my $file = $self->_get_file($name) or next;
        if ($file->[PATH] && $file->[SIZE] <= $self->inline_max) {
            $buffer .= $self->inline_html($name, $file);
        elsif ($self->defer_css) {
            my $href = $self->href($name, $file);
            push @deferred, $href;
            $buffer .= $self->preload_template->($href);
        else {
            $buffer .= $self->link_html($name, $file);

    if (@deferred) {

        $buffer .= "<noscript>" .
            join("", map { $self->link_template->($_) } @deferred ) .
            "</noscript>" if $self->include_noscript;

        $buffer .= "<script>" .
            $self->preload_script->slurp_raw .
            "</script>" unless $self->simple;


    return $buffer;

sub _get_file {
    my ($self, $name) = @_;
    unless (defined $name) {
        $self->log->( error => "missing name" );
    if (my $file = $self->css_files->{$name}) {
        return $file;
    else {
        $self->log->( error => "invalid name '$name'" );





=encoding UTF-8

=head1 NAME

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

=head1 VERSION

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 L<JavaScript|>
to handle this, as well as a C<noscript> block as a fallback.

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


=item *

declaring all stylesheets used by your web application;

=item *

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

=item *

enable or disable the use of minified stylesheets;

=item *

switch between local copies of stylesheets or CDN versions;

=item *

automatically inline small stylesheets;

=item *

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



=head2 aliases

This is a required hash reference of names and their relative
filenames to L</css_root>.

It is recommended that the F<.css> and F<.min.css> suffixes be

If the name is the same as the filename (without the extension) than
you can simply use C<1>.  (Likewise, an empty string or C<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.

=head2 css_root

This is the required root directory where all stylesheets can be

=head2 url_base_path

This is the URL prefix for stylesheets.

It can be a full URL prefix.

=head2 prefer_min

If true (default), then a file with the F<.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.

=head2 css_files

This is a hash reference used internally to translate L</aliases>
into the actual files or URLs.

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

=head2 cdn_links

This is a hash reference of L</aliases> to URLs. (Only one URL per
alias is supported.)

When L</use_cdn_links> is true, then these URLs will be used instead
of local versions.

=head2 has_cdn_links

This is true when there are L</cdn_links>.

=head2 use_cdn_links

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

=head2 inline_max

This specifies the maximum size of an file to inline.

Local files under the size will be inlined using the
L</link_or_inline_html> or L</deferred_link_html> methods.

Setting this to 0 disables the use of inline links, unless
L</inline_html> is called explicitly.

=head2 defer_css

True by default.

This is used by L</deferred_link_html> to determine whether to emit
code for deferred stylesheets.

=head2 include_noscript

When true, a C<noscript> element will be included with non-deffered

This defaults to the same value as L</defer_css>.

=head2 preload_script

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

The script comes from L<>.

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

=head2 link_template

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

=head2 preload_template

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

=head2 asset_id

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.

=head2 has_asset_id

True if there is an L</asset_id>.

=head2 log

This is a code reference for logging errors and warnings:

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

By default, this is a wrapper around L<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(@_) },

=head2 simple

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

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

See L<>.

This attribute was added in v0.4.0.

=head1 METHODS

=head2 check

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

It will throw an error or return false (depending on L</log>) if there
is something wrong.

This was added in v0.3.0.

=head2 href

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

This returns this URL for an alias.

=head2 link_html

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

This returns the link HTML markup for the stylesheet referred to by

=head2 inline_html

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

This returns an embedded stylesheet referred to by C<$alias>.

=head2 link_or_inline_html

  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 L</inline_max>.

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

=head2 deferred_link_html

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

This returns the HTML markup for the stylesheets specified by
L</aliases>, as appropriate for each stylesheet.

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

=for Pod::Coverage PATH NAME SIZE


=head2 Content-Security-Policy (CSP)

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

=head2 XHTML Support

This module is written for HTML5.

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

=head2 Encoding

All files are embedded as raw files.

No URL encoding is done on the HTML links or L</asset_id>.

=head2 It's spelled "Deferrable"

It's also spelled "Deferable".

=head1 SOURCE

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

=head1 BUGS

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

Please report any bugs in F<cssrelpreload.js> to

=head1 AUTHOR

Robert Rothenberg <>

This module was developed from work for Science Photo Library

F<reset.css> comes from L<>.

F<cssrelpreload.js> comes from L<>.


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

This is free software, licensed under:

  The MIT (X11) License