# NAME

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

# VERSION

version v0.4.2

# SYNOPSIS

```perl
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  => '//cdn.example.com/jquery-ui.min.css',
    },
);

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

...

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

# DESCRIPTION

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](https://github.com/filamentgroup/loadCSS)
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.

# ATTRIBUTES

## aliases

This is a required hash reference of names and their relative
filenames to ["css\_root"](#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:

```perl
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:

```perl
my $css = HTML::DeferableCSS->new(
  aliases => [ qw( foo bar } ],
  ...
);
```

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

```
$css->deferred_link_html('gone')
```

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.

## css\_root

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

## url\_base\_path

This is the URL prefix for stylesheets.

It can be a full URL prefix.

## prefer\_min

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.

## css\_files

This is a hash reference used internally to translate ["aliases"](#aliases)
into the actual files or URLs.

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

## cdn\_links

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

When ["use\_cdn\_links"](#use_cdn_links) is true, then these URLs will be used instead
of local versions.

## has\_cdn\_links

This is true when there are ["cdn\_links"](#cdn_links).

## use\_cdn\_links

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

## inline\_max

This specifies the maximum size of an file to inline.

Local files under the size will be inlined using the
["link\_or\_inline\_html"](#link_or_inline_html) or ["deferred\_link\_html"](#deferred_link_html) methods.

Setting this to 0 disables the use of inline links, unless
["inline\_html"](#inline_html) is called explicitly.

## defer\_css

True by default.

This is used by ["deferred\_link\_html"](#deferred_link_html) to determine whether to emit
code for deferred stylesheets.

## include\_noscript

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

This defaults to the same value as ["defer\_css"](#defer_css).

## preload\_script

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

The script comes from [https://github.com/filamentgroup/loadCSS](https://github.com/filamentgroup/loadCSS).

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

## link\_template

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

## preload\_template

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

## 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.

## has\_asset\_id

True if there is an ["asset\_id"](#asset_id).

## log

This is a code reference for logging errors and warnings:

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

By default, this is a wrapper around [Carp](https://metacpan.org/pod/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,

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

or that warnings are fatal,

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

or even integrate this with your own logging system:

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

## simple

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.

See [https://www.filamentgroup.com/lab/load-css-simpler/](https://www.filamentgroup.com/lab/load-css-simpler/).

This attribute was added in v0.4.0.

# METHODS

## check

This method instantiates lazy attributes and performs some minimal
checks on the data.  (This should be called instead of ["css\_files"](#css_files).)

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

This was added in v0.3.0.

## href

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

This returns this URL for an alias.

## link\_html

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

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

## inline\_html

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

This returns an embedded stylesheet referred to by `$alias`.

## link\_or\_inline\_html

```perl
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"](#inline_max).

Note that a stylesheet will be inlined, even if there is are
["cdn\_links"](#cdn_links).

## deferred\_link\_html

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

This returns the HTML markup for the stylesheets specified by
["aliases"](#aliases), as appropriate for each stylesheet.

If the stylesheets are not greater than ["inline\_max"](#inline_max), then it will
embed them.  Otherwise it will return the appropriate markup,
depending on ["defer\_css"](#defer_css).

# KNOWN ISSUES

## Content-Security-Policy (CSP)

If a web site configures a
[Content-Security-Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP)
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.

## Encoding

All files are embedded as raw files.

No URL encoding is done on the HTML links or ["asset\_id"](#asset_id).

## It's spelled "Deferrable"

It's also spelled "Deferable".

# SOURCE

The development version is on github at [https://github.com/robrwo/HTML-DeferableCSS](https://github.com/robrwo/HTML-DeferableCSS)
and may be cloned from [git://github.com/robrwo/HTML-DeferableCSS.git](git://github.com/robrwo/HTML-DeferableCSS.git)

# BUGS

Please report any bugs or feature requests on the bugtracker website
[https://github.com/robrwo/HTML-DeferableCSS/issues](https://github.com/robrwo/HTML-DeferableCSS/issues)

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
[https://github.com/filamentgroup/loadCSS/issues](https://github.com/filamentgroup/loadCSS/issues).

# AUTHOR

Robert Rothenberg <rrwo@cpan.org>

This module was developed from work for Science Photo Library
[https://www.sciencephoto.com](https://www.sciencephoto.com).

`reset.css` comes from [http://meyerweb.com/eric/tools/css/reset/](http://meyerweb.com/eric/tools/css/reset/).

`cssrelpreload.js` comes from [https://github.com/filamentgroup/loadCSS/](https://github.com/filamentgroup/loadCSS/).

# COPYRIGHT AND LICENSE

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

This is free software, licensed under:

```
The MIT (X11) License
```
