ki premailer-rails

CSS styled emails without the hassle.

Introduction

This gem is a drop in solution for styling HTML emails with CSS without having to do the hard work yourself.

Styling emails is not just a matter of linking to a stylesheet. Most clients, especially web clients, ignore linked stylesheets or <style> tags in the HTML. The workaround is to write all the CSS rules in the style attribute of each tag inside your email. This is a rather tedious and hard to maintain approach.

Premailer to the rescue! The great premailer gem applies all CSS rules to each matching HTML element by adding them to the style attribute. This allows you to keep HTML and CSS in separate files, just as you're used to from web development, thus keeping your sanity.

This gem is an adapter for premailer to work with actionmailer out of the box. Actionmailer is the email framework used in Rails, which also works outside of Rails. Although premailer-rails has certain Rails specific features, it also works in the absence of Rails making it compatible with other frameworks such as sinatra.

How It Works

premailer-rails works with actionmailer by registering a delivery hook. This causes all emails that are delivered to be processed by premailer-rails. This means that by simply including premailer-rails in your Gemfile you'll get styled emails without having to set anything up.

Whenever premailer-rails processes an email, it collects the URLs of all linked stylesheets (<link rel="stylesheet" href="css_url">). Then, for each of these URLs, it tries to get the content through a couple of strategies. As long as a strategy does not return anything, the next one is used. The strategies and their order are as follows:

1. Cache: If there's a file in cache matching that URL, the cache content is returned. The cache right now is rather rudimentary. Whenever a CSS file is retrieved, it is stored in memory such that subsequent requests to the same file are faster. The caching is disabled inside Rails in the development environment.

2. File System: If there's a file inside public/ with the same path as in the URL, it is read from disk. E.g. if the URL is http://cdn.example.com/assets/email.css the contents of the file located at public/assets/email.css gets returned if it exists.

3. Asset Pipeline: If Rails is available and the asset pipeline is enabled, the file is retrieved through the asset pipeline. E.g. if the URL is http://cdn.example.com/assets/email-fingerprint123.css, the file email.css is requested from the asset pipeline. That is, the fingerprint and the prefix (in this case assets is the prefix) are stripped before requesting it from the asset pipeline.

4. Network: As a last resort, the URL is simply requested and the response body is used. This is usefull when the assets are not bundled in the application and only available on a CDN. On Heroku e.g. you can add assets to your .slugignore causing your assets to not be available to the app (and thus resulting in a smaller app) and deploy the assets to a CDN such as S3/CloudFront.

Installation

Simply add the gem to your Gemfile:

gem 'premailer-rails'

premailer-rails requires either nokogiri or hpricot. It doesn't list them as a dependency so you can choose which one to use. Since hpricot is no longer maintained, I suggest you to go with nokogiri. Add either one to your Gemfile:

gem 'nokogiri'
# or
gem 'hpricot'

If both gems are loaded for some reason, premailer chooses hpricot.

That's it!

Configuration

Premailer itself accepts a number of options. In order for premailer-rails to pass these options on to the underlying premailer instance, specify them as follows (in Rails you could do that in an initializer such as config/initializers/premailer_rails.rb):

Premailer::Rails.config.merge!(preserve_styles: true, remove_ids: true)

For a list of options, refer to the premailer documentation. The default configs are:

{
  input_encoding: 'UTF-8',
  generate_text_part: true
}

If you don't want to automatically generate a text part from the html part, set the config :generate_text_part to false.

Note that the options :with_html_string and :css_string are used internally by premailer-rails and thus will be overridden.

If you're using this gem outside of Rails, you'll need to call Premailer::Rails.register_interceptors manually in order for it to work. This is done ideally in some kind of initializer, depending on the framework you're using.

Usage

premailer-rails processes all outgoing emails by default. If you wish to skip premailer for a certain email, simply set the :skip_premailer header:

class UserMailer < ActionMailer::Base
  def welcome_email(user)
    mail to: user.email,
         subject: 'Welcome to My Awesome Site',
         skip_premailer: true
  end
end

Note that the mere presence of this header causes premailer to be skipped, i.e., even setting skip_premailer: false will cause premailer to be skipped. The reason for that is that the skip_premailer is a simple header and the value is transformed into a string, causing 'false' to become truethy.

Small Print

Author

Philipe Fatio (@fphilipe)

License

premailer-rails is released under the MIT license. See the license file.