Saturday, October 30, 2010

CSS Specificity And Inheritance

CSS’ barrier to entry is extremely low, mainly due to the nature of its syntax. Being clear and easy to understand, the syntax makes sense even to the inexperienced Web designer. It’s so simple, in fact, that you could style a simple CSS-based website within a few hours of learning it.

But this apparent simplicity is deceitful. If after a few hours of work, your perfectly crafted website looks great in Safari, all hell might break loose if you haven’t taken the necessary measures to make it work in Internet Explorer. In a panic, you add hacks and filters where only a few tweaks or a different approach might do. Knowing how to deal with these issues comes with experience, with trial and error and with failing massively and then learning the correct way.

Understanding a few often overlooked concepts is also important. The concepts may be hard to grasp and look boring at first, but understanding them and knowing how to take advantage of them is important.

Two of these concepts are specificity and inheritance. Not very common words among Web designers, are they? Talking about border-radius and text-shadow is a lot more fun; but specificity and inheritance are fundamental concepts that any person who wants to be good at CSS should understand. They will help you create clean, maintainable and flexible style sheets. Let’s look at what they mean and how they work.

The notion of a “cascade” is at the heart of CSS (just look at its name). It ultimately determines which properties will modify a given element. The cascade is tied to three main concepts: importance, specificity and source order. The cascade follows these three steps to determine which properties to assign to an element. By the end of this process, the cascade has assigned a weight to each rule, and this weight determines which rule takes precedence, when more than one applies.

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