File Size Optimization

Compressing CSS and JavaScript

Middleman handles CSS minification and Javascript compression so you don't have to worry about it. Most libraries ship minified and compressed versions of their files for users to deploy, but these files are unreadable or editable. Middleman allows you to keep the original, commented files in our project so you can easily read them and edit them if needed. Then, when you build the project, Middleman will handle all the optimization for you.

In your config.rb, activate the minify_css and minify_javascript features during the build of your site.

configure :build do
  activate :minify_css
  activate :minify_javascript

If you are already using a compressed file that includes .min in its filename, Middleman won't touch it. This can be good for libraries like jQuery which are carefully compressed by their authors ahead of time.

You can customize how the JavaScript compressor works by setting the :compressor option for the :minify_javascript extension in config.rb to a custom instance of Uglifier. See Uglifier's docs for details.

For example, you could enable unsafe optimizations and mangle top-level variable names like this:

require "uglifier"
activate :minify_javascript,
  compressor: proc { => {:toplevel => true}, :compress => {:unsafe => true})

If you have asset_hash activated, are building your site on multiple servers during deploy to sit behind a load balancer, and are compressing Javascript, ensure that mangling variables is disabled. If mangling is enabled, Uglifier will create different compressed versions of the Javascript on each machine, leading to different hashes in the filename and different references in each version of the HTML. For example:

require "uglifier"
activate :minify_javascript, compressor: -> { => false) }

If you want to exclude any files from being minified, pass the :ignore option when activating these extensions, and give it one or more globs, regexes, or procs that identify the files to ignore. Likewise, you can pass an :exts option to change which file extensions are renamed.

You can speed up your JavaScript minification (and CoffeeScript builds) by including these gems in your Gemfile:

gem 'therubyracer' # faster JS compiles
gem 'oj' # faster JS compiles

GZIP text files

It's a good idea to serve compressed files to user agents that can handle it. Many web servers have the ability to gzip files on the fly, but that requires CPU work every time the file is served, and as a result most servers don't perform the maximum compression. Middleman can produce gzipped versions of your HTML, CSS, and JavaScript alongside your regular files, and you can instruct your web server to serve those pre-gzipped files directly. First, enable the :gzip extension:

activate :gzip

Then configure your server to serve those files. If you use Nginx, check out the gzip_static module. For Apache, you'll have to do something a little trickier - see this Gist for an example.

Compressing Images

If you also want to compress images on build, try middleman-imageoptim.

Minify HTML

Middleman provides an official extension for minifying its HTML output. Simply install the gem:

gem install middleman-minify-html

Add middleman-minify-html to your Gemfile:

gem "middleman-minify-html"

Then open your config.rb and add:

activate :minify_html

You should notice whilst view-source:'ing that your HTML is now being minified.

Using source sets

One of the more recent additions to HTML is the srcset attribute for the img or picture tag. It allows you to define for the browser to load different images with different sizes dependent on either the viewport (using width such as 1024w, 800w, 600w, or 320w) or the resolution of the current browser display (using factors 1x, 2x, 3x, ...).

<img src="img/100px.jpg" srcset="img/300px.jpg 3x, img/200px.jpg 2x, img/100px.jpg 1x">
<img src="img/100px.jpg" srcset="img/300px.jpg 300w, img/200px.jpg 200w, img/100px.jpg 100w">

If you want to use srcset in conjunction with the :asset_hash option, you need to employ the image_path helper, which is described in this middleman section:

<img src="<%= image_path('100px.jpg') %>" srcset="<%= image_path('300px.jpg') %> 3x, <%= image_path('200px.jpg') %> 2x, <%= image_path('100px.jpg') %> 1x">

The srcset attribute is not yet supported by all browsers as you can see at If it is not supported the browser uses the src attribute as fallback. We noted, that some browsers use the first entry of the srcset as fallback. This is why we have put the largest image first in our example.