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In CSS Level 2.1 you may collect attribute values and use them as content only for the pseudo-elements. For instance, the :before pseudo-element can be used to insert some content before an element. This is valid in CSS 2.1:

title:before{
  content: "[Audience Level: " attr(audience) "]";
}

If the title element from the XML document is:

<title audience="Expert">Changing the Timing Belt</title>

Then the title will be displayed as:

[Audience Level: Expert] Changing the Timimg Belt

In Oxygen XML Editor, the use of attr() function is available not only for the content property, but also for any other property. This is similar to the CSS Level 3 working draft: http://www.w3.org/TR/2006/WD-css3-values-20060919/#functional. The arguments of the function are:

attr ( attribute_name , attribute_type , default_value )
attribute_name

The attribute name. This argument is required.

attribute_type

The attribute type. This argument is optional. If it is missing, argument's type is considered string. This argument indicates what is the meaning of the attribute value and helps to perform conversions of this value. Oxygen XML Editor accepts one of the following types:

color

The value represents a color. The attribute may specify a color in various formats. Oxygen XML Editor supports colors specified either by name (red, blue, green, etc.) or as an RGB hexadecimal value #FFEEFF.

url

The value is a URL pointing to a media object. Oxygen XML Editor supports only images. The attribute value can be a complete URL, or a relative one to the XML document. Note that this URL is also resolved through the catalog resolver.

integer

The value must be interpreted as an integer.

number

The value must be interpreted as a float number.

length

The value must be interpreted as an integer.

percentage

The value must be interpreted relative to another value (length, size) expressed in percents.

em

The value must be interpreted as a size. 1 em is equal to the font-size of the relevant font.

ex

The value must be interpreted as a size. 1 ex is equal to the height of the x character of the relevant font.

px

The value must be interpreted as a size expressed in pixels relative to the viewing device.

mm

The value must be interpreted as a size expressed in millimeters.

cm

The value must be interpreted as a size expressed in centimeters.

in

The value must be interpreted as a size expressed in inches. 1 inch is equal to 2.54 centimeters.

pt

The value must be interpreted as a size expressed in points. The points used by CSS2 are equal to 1/72th of an inch.

pc

The value must be interpreted as a size expressed in picas. 1 pica is equal to 12 points.

default_value

This argument specifies a value that is used by default if the attribute value is missing. This argument is optional.

Example: attr Function

Consider the following XML instance:
<sample>
   <para bg_color="#AAAAFF">Blue paragraph.</para>
   <para bg_color="red">Red paragraph.</para>
   <para bg_color="red" font_size="2">Red paragraph with large font.</para>
   <para bg_color="#00AA00" font_size="0.8" space="4">
        Green paragraph with small font and margin.</para>
</sample>
The para elements have bg_color attributes with RGB color values (such as #AAAAFF). You can use the attr() function to change the elements appearance in the editor based on the value of this attribute:
background-color:attr(bg_color, color);
The attribute font_size represents the font size in em units. You can use this value to change the style of the element:
font-size:attr(font_size, em);
The complete CSS rule is:
para{
 display:block;
 background-color:attr(bg_color, color);
 font-size:attr(font_size, em);
 margin:attr(space, em);
}

The document is rendered as: