How to Check Which CSS Styles Are Being Used or Not Used on a Web Page

Is there a way to check which CSS styles are being used or not used on a web page?

Install the CSS Usage add-on for Firebug and run it on that page. It will tell you which styles are being used and not used by that page.

Find out what CSS styles are being used on a given page

I think this website does what you want: CSS Trashman. It takes a little while to run, but it works. It reduced my personal site's CSS from 3.31 KB to 402 Bytes.

If you would rather use a command-line tool, CSS Rationator powers CSS Trashman.

Find out where the CSS codes are used in the web page after Pick out them via Chrome DevTools?

Hei, instead of going to the source options, go in the Elements tab and look over the rule you are searching for, press on it and on the right you can see which file applies that style, and at which row. Like the screenshot below (just an example)
Example

EDIT

2 CSS files use the same class name for the same CSS rule, how to find such a case?

Well, there are cases where multiple CSS files could apply style at the same class element. This divides into 2 categories:

The one that applies different styles to the same class
The one that applies the same styles to the same class

In the first case, there are no issues, and both styles will be applied. And to track those styling rules, you'd need to check both files.

In the second case, the style will be applied by the last rule, so the last read by the system (example below)
You can see how it works

Also, on the second posted screenshot, you can see that the element that has "no applied style" simply does not exist, and is not a real element but a string
noscript
You can see in this screenshot that the image element is written between double quotes and does not represent a real HTML element. so no styling will be applied

How to find which CSS files are used for current page

Use http://getfirebug.com/ to debug the page.

When looking at the css it will reference which stylesheet is used.

For example:

You can see from the screenshot that when using Firebug it shows in the blue text that the page is using the style.css stylesheet.

Sample Image

Work down the tree in Firebug and simply hover over your elements and see which stylesheets are used.

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 ^^)

Programmatically check whether a CSS file is used

There is no way to check, whether file itself is used. Browser will load them all. But what you can do is programmatically check is a selector is used. Therefore, if each of your CSS files has one unique rule, you can exploit it.

In each file add

UNIQUE_RULE::after {
    content: ' ';
    background: url(/track/?page=filename.css);
}

_{Note: I mentioned using unique rule, because this approach does not let you check overrides.}

Browser loads background images only for tags, that exist in the DOM tree. For this file to be used, you need to use on of its rules. Then you will need is to set up Selenium(or its alternative) to "click" through all of your pages.

After your test has finished running, you just grep through your access logs and look for the missing /track/ file requests. And you can use this same approach to check each individual CSS rule (by making script to add ::after{ .. } to each rule) too with no changes to the Selenium setup.

It will require some time investment, but, when you have set it all up, it will be possible to reuse it.

Not an elegant solution, but workable.

Extracting only the css used in a specific page

I've used Dust-Me Selectors before, which is a Firefox plugin. It's very easy to use and versatile as it maintains a combined list across a number of pages of which CSS values are used.

The downside is that I wasn't able to automate it to spider an entire site, so I ended up using it just on key pages/templates of my site. It is very useful nonetheless.

http://www.sitepoint.com/dustmeselectors/

https://addons.mozilla.org/en-US/firefox/addon/dust-me-selectors/

Contrary to the comment above Dust-Me Selectors 2.2 is compatible with Firefox 3.6 (I've just installed it).

Find all CSS styles used on website

For firefox there is an add-in called dust-me-selectors. If you provide a sitemap, it will find all unused css styles.