How can I extract only the used CSS on a given web page and have that combined into a separate style sheet?

(deleted my comment to RwwL answer to make it a thorough answer)

UnCSS, whether node.js or as a grunt or gulp task, is able to list used CSS rules by an array of pages in an array of Media Queries.

uncss: https://github.com/giakki/uncss
grunt-uncss: https://github.com/addyosmani/grunt-uncss
gulp-uncss: https://github.com/ben-eb/gulp-uncss

Multipage:

You can pass files as an argument to any of the 3 plugins, like:

var files   = ['my', 'array', 'of', 'HTML', 'files'],
    options = { /* (…) */ };

uncss(files, options, function (error, output) {
    console.log(output);
});

Avoid:

urls (Array):
array of URLs to load with Phantom (on top of the files already passed if any).
NOTE: this feature is deprecated, you can pass URLs directly as arguments.

 
Media Queries and responsive are taken into account:

media (Array):
By default UnCSS processes only stylesheets with media query "all", "screen", and those without one. Specify here which others to include.

You can add stylesheets, ignore some of them, add inline CSS and many other options like htmlroot

 
Remaining problems:

1/ Conditional classes if you use them for IE9-. They obviously won't be matched in a WebKit PhantomJS environment! 

HTML:
<!--[if IE 9]><html class="ie9 lte-ie9" lang="en"><![endif]--> <!-- teh conditional comment/class -->

CSS:
.ie9 .some-class { property: value; ] /* Only matched in IE9, not WebKit PhantomJS */

Should they be added by hand or script to the html element in testing environment? (how it renders is of no importance)
Is there an option in uncss?
As long as you don't style with :not(.ie9) (weird), it should be fine.

EDIT: you can use the ignore option with a pattern to force uncss to "provide a list of selectors that should not be removed by UnCSS". Won't be tested though.

2/ Scripts that will detect resolution (viewport width) and adapt content to it by removing/adding it or adding a class on a container. They will execute in PhantomJS in desktop resolution I guess and thus won't do their job so you'll need to modify calls to PhantomJS or something like that... Or dig into options or GitHub issues of the 3 projects (I didn't)

Other tools I heard of, not tested or barely or couldn't test, no idea about the MQ part:

  • in grunt-uncss readme, the Coverage part
  • ucss from Opera (there's already an ansswer here, couldn't make it work)
  • Helium
  • CSSESS
  • mincss

Addy Osmani has countless presentations of 100+ slides presenting awesome tools like this one: https://speakerdeck.com/addyosmani/automating-front-end-workflow (you'll regret even more that days are made only of 24 hours and not 48 err wait 72 ^^)


How about the CSS Usage plugin for Firebug?

Steps:

  1. Visit your page in Firefox
  2. Click "CSS Usage" tab in Firebug
  3. Click the Scan button
  4. Click the bold file name
  5. Save page of CSS selectors to disk

Here are some screen shots and walk through. Not sure about media queries or if it'll work on your site, and it'll probably not keep -webkit etc, but maybe it'll get you part of the way there.


Opera Software released a CSS crawler on Github that claims it can find unused and duplicate selectors. It might do the trick if you're comfortable with a command-line tool. https://github.com/operasoftware/ucss

Tags:

Css

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