CSS Validation with Visual Studio Web Essentials

_This is a cross post of an _article over on the Canadian Developer Connection.

I like when my IDE tells me I’m doing something wrong. I find it mush less embarrassing than when a co-worker or customer tells me I’m doing something wrong. That’s why I love Visual Studio and the Web Essentials extension. Web Essentials has some great CSS Validation features that can help to point out common mistakes. These features also apply to LESS.

There are a number of useful validation rules and best practices that Web Essentials will check. In this post, we will explore those that I found particularly useful and taught me a little more about CSS.

Vendor Prefixes

Vendor prefixes are used by browser vendors to add new CSS features that may not yet been finalized in the official CSS specs. Each browser has it’s own prefix for CSS properties. Chrome and Safari use –webkit, Firefox uses –moz and Internet Explorer uses  -ms.

For example, if you want to use CSS Column layouts, you will need to specify vendor prefixes as follows:

-moz-column-count: 2;
-webkit-column-count: 2;
column-count: 2;

Unless you follow the CSS specs and individual browser implementations closely, it can be very difficult to know when you need to use vendor prefixes. This is where Web Essentials can help.  If I specify the a column-count property without the required browser prefixes, it will be flagged as a warning. Web Essentials shows warnings by underlining items in the CSS editor and by listing them in the Error List (View –> Error List).

When I select the Add missing vendor specifics smart tag, Web Essentials will add the required prefixes for me.

Now, the CSS specs and browser implementations are always changing. While the –moz and –webkit prefixes are required today, they may not be required tomorrow. A good example of this is border-radius. In the past, vendor prefixes were required to support border-radius. Today, border-radius is officially part of the CSS specs and is supported by all major browsers. Web Essentials will point out that these vendor specifics are not longer necessary.

For Web Essentials to warn you of deprecated vendor specifics, you may need to set the Disallow unrecognized vendor-specifics setting to True in the Web Essentials settings. You can find Web Essentials settings under Tools –> Options. In the options dialog, select Web Essentials –> CSS.

Choosing your Browsers

Maybe your website is deployed internally and you have no need to support a particular browser. You can tell Web Essentials what browsers you are targeting by right clicking in any CSS or LESS file and selecting Web Essentials –> Select Browsers.

In this case, I un-checked FireFox. When I click Ok, Web Essentials adds a file ‘WE-browsers.xml’ to my solution. If you include this file in source control, the settings will be shared across all developers on your team.

Now, when I ask Web Essentials to add missing vendor specifics, the –moz prefix is not included.

Duplicate Properties and Selectors

As a CSS file grows over time, it can be common for a developer to accidently define the same selector more than once in a CSS file. Web Essentials will scan a the file and warn you if this happens. It will also check to make sure that you have not accidently defined the same property more than once within a selector.


We only scratched the surface of the validation rules that are available in Web Essentials. If you are interested in a complete list, you can read more on the Web Essentials site.

If you are interested in the implementation details, you can view the source code on GitHub. Remember, Web Essentials is an open source project, so you can contribute too!