cssert – pronounced however you feel like it – is my attempt at bringing some kind of style verification into an automated build process. If you've read the previous article, you'll know that this falls into the second group of CSS test frameworks, style measurement and comparison. The system works exactly as I described above – your test files have a basic HTML structure created by traversing the DOM from the element being tested upwards. You could also include your entire HTML in the test file if you liked, it would just be unnecessary in most cases.

I've created a couple of (for me, at least) helpful utilities which allow these test cases to be generated from a page via a bookmarklet and then run in the browser or on the command-line. Running the tests in the browser is useful for quick human verification while the command-line interface can be integrated into an automated build system if you like that kind of thing. The test file structure is quite simple (all samples here taken from the Twitter Bootstrap project:

First, we have the test file opening structure:

<!doctype html><html><head><title>cssert test page</title><link rel="stylesheet" href="../lib/cssert.css"></head><body><h1>cssert Test cases</h1><p>click to expand test</p><script type="text/html">/*==

Then we have the name of the test:

Intro Paragraph

Then we have the skeleton DOM:

<!doctype html><html><head><meta charset="utf-8"><base href="http://twitter.github.com/bootstrap/examples/hero.html"><link href="../1.3.0/bootstrap.css" rel="stylesheet"><style type="text/css">
      body {
        padding-top: 60px;
    </style><style>#cssert-style-modal {display:none;position: fixed;top: 10%;left: 50%;margin-left: -350px;width: 700px;background: #39c;color: white;padding: 10px;color: #fff;text-shadow: 0 1px 0 rgba(0,0,0,.3);background-image: -webkit-linear-gradient(-45deg, rgba(255,255,255,0), rgba(255,255,255,.1) 60%, rgba(255,255,255,0) 60%);border-radius: 5px;border: 1px solid #17a;box-shadow: inset 0 0 0 1px rgba(255,255,255,.3);}#cssert-style-modal ul,#cssert-style-modal li {margin:0;padding:0;font-size:11px;list-style:none;}#cssert-style-modal>ul>li {float:left;width:140px;font-size:13px;}#cssert-style-modal ul {margin-bottom:10px;}#cssert-pre {position:fixed;top:10px;right:10px;background:white;border:1px solid gray;width:200px;height:200px;overflow:auto;}#cssert-drag {position:fixed;top:210px;right:10px;background:white;border:1px solid gray;width:200px;height:20px;overflow:auto;}</style></head><body><div class="container"><div class="hero-unit"><p>Vestibulum id ligula porta felis euismod semper. Integer posuere erat a ante venenatis dapibus posuere velit aliquet. Duis mollis, est non commodo luctus, nisi erat porttitor ligula, eget lacinia odio sem nec elit.</p></div></div></body></html>

The CSS selector identifying the element to verify:

html body div div p

And the styles we wish to verify:

{"font-family":"'Helvetica Neue', Helvetica, Arial, sans-serif","font-weight":"200","font-style":"normal","color":"rgb(64, 64, 64)","text-transform":"none","text-decoration":"none","letter-spacing":"normal","word-spacing":"0px","line-height":"27px","text-align":"-webkit-auto","vertical-align":"baseline","direction":"ltr"}

A test file can contain as many test units as you like. At the very end is the close of the test file structure

*/</script><script src="../lib/cssert.js"></script></body></html>

You'll probably notice the crucial bit in the test structure is the base element. The CSS available from the location specified here is the thing we are actually testing. In typical test lingo, the structure we have in our test file is the mock and our tests work by asserting the values ‘output’ from applying the CSS to this structure are as expected.

Running the tests

Running the tests in-browser

Open the test page in a browser. That's it. If it's green and says 'passed', the test passed. If it's red and says 'failed', the test failed. You can see the output by clicking on the title of the test.

This works by loading the test file, creating an iframe and injecting the test case into the iframe as source. It then looks into the iframe and measures the styles. If they match those specified in the test file, it passes, otherwise, it fails. Clicking on the test title simply removes the position:absolute which is hiding the iframe.

Running the tests on command-line

The exact same test page can also be used with the command-line interface. cssert uses PhantomJS to run the tests in a headless webkit instance. You'll need to install PhantomJS into your path after downloading it. Place your test case in the tests folder and run:

$ ./cssert testcase.html

To run all tests in the tests folder at once, simply run with no arguments:

$ ./cssert

This works by, again, loading the HTML from the test files. In this case, the structure is injected into a new headless browser window. The styles are measured and the output is redirected to stdout. Each browser window test is also rendered as a PNG so you can see what failed if any did.


I'm not saying this is the ultimate solution to CSS testing. Declarative languages don't sit well with testing. This is as close as I can get for the moment. I'm also not going to be able to head off or counter all possible complaints or criticisms but I will cover a couple.

Firstly, most of the limitations you'll run into are caused by using the automatically generated tests. They're good for creating a starting point but at the moment, they need to be tweaked for many cases.

Sibling selectors

Because the test generates the DOM via following parents up the structure, sibling elements are ignored. These styles are still testable, though. Simply add the sibling element into your HTML test block.

Styles modified by JS

The styles are measured on the element as it is when the case is generated. The test compares this against the styles provided by the CSS. If the element contains JS-only styles not added by CSS, they will not be correctly matched. Modify your test case to allow for this.

Why not use Selenium?

This, essentially does the same as Selenium would do if you took the time to set up your test cases. This makes it much easier to set up test cases, though.


If your @font-face declaration contains a suggested 'local' source (as recommended in Paul Irish's bulletproof syntax), a bug in QTWebkit will prevent the test case from running correctly.


Just clone the git project from git@github.com:thingsinjars/cssert.git and you're good to go.

The tests directory comes with some sample tests generated using Twitter's Bootstrap project. Put your tests in that same place.