Explanation of Terms

This reference uses CSS-specific terminology in its headings within a property description. This article describes these terms and explains their meanings and possible values.


Syntax describes the syntax of a CSS property. If a property can have multiple forms, each form appears in its own line.

Types Allowed

Types Allowed contains information about what numeric types are allowed in a given property. This field is omitted from properties that do not have a single numeric type, such as properties that take multiple values or use nonnumeric constants exclusively.

The Types Allowed field does not present the complete story, however, because a property may also take additional types specific to its subproperties.


Constants contains a list of special nonnumeric values that you can assign to a specific property. For example, the border-width property can take the value caption.

The Constants field does not present the complete story, however, because a property may also take additional values specific to its subproperties.


Subproperties provides a list of properties that make up a larger property.

There are three basic types of properties: simple properties, convenience properties, and composite properties.

Convenience properties, such as border-width, have related subproperties with finer granularity. For example, instead of setting the border-width property, you could set the border-bottom-width, border-top-width, border-left-width, and border-right-width properties to the same value and achieve the same result.

Because these convenience properties can be broken down into subproperties of the same basic type, any value that is legal for all of the subproperties is also legal for the convenience property as a whole if the property has a single-value form, and for the individual parts if the property has a multiple-value form. For example, the border-width property can accept the value thin even though it is listed only in related subproperties such as border-bottom-width. Similarly, you could use a multiple-value form, such as border-width: thin thin thin thin.

Composite properties also have related subproperties. For example, the second parameter in the border property is equivalent to the border-width property. Thus, any value that is appropriate for the border-width property is also appropriate for the width portion of the border composite property.

Similarly, the types allowed for a subproperty are also allowed for convenience properties and the appropriate portions of composite properties that contain them.


Availability gives the version of Safari in which the property first appeared. If no availability is listed you can assume the property has been supported for both Safari and iOS for several releases.

Support Level

Support Level specifies the revision of the W3C standard in which a given property is defined, where applicable, and provides the overall status of the property for properties that are not part of a W3C standard. The possible values are:

Properties may be upgraded to more stable categories as time passes, particularly as tags are added to relevant standards. Go to http://webkit.org/specs for current W3C proposals.