Sunday, June 28, 2009

Do not put experimental features last in your CSS

Today's blog post will be short one. I have lately seen code like this on more than one occasion:

#foo { 
border-radius : 10px; 
-moz-border-radius : 10px; /* Mozilla */
-webkit-border-radius : 10px; /* Webkit */
}

What is the problem?

Right now only one line will have an effect. In Gecko-based browsers, like Firefox, the one starting with -moz-, in recent Webkit based browsers, like Safari and Chrome, the one starting with -webkit-, as indicated in the comments. These are the two current experimental implementations of the coming CSS 3 border radius property.

Hopefully in the near future the specification will thanks to these two implementations reach a level of maturity, where browsers may start to implement it in a non-experimental version. If so one thing is to be expected. The non-experimental implementation will most likely differ (somewhat) from the experimental one! And as a developer you will most probably want the final version to be the one that browsers actually use. And when two rules affect the same property like this (equal specificity, equally placed in the cascade), the last one will override the first one. Therefore you should put things in this order:

#foo { 
-moz-border-radius : 10px; /* Mozilla */
-webkit-border-radius : 10px; /* Webkit */
border-radius : 10px; 
}

Why is this better?

During a transition phase, lasting at least a few years, Webkit and Gecko can be expected to support both implementations. There are sites that use the experimental versions only and in order to give them a grace period, dropping support as soon as the final version is implemented is not an option. Historically Mozilla has let such grace periods last for 2-4 versions of Firefox.

So this is the bottom line. Put experimental features first, standard features last. That will ensure a better forward compatibility.

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