In CSS, a certain object can only have one instance of a property. A `<div>` has a specific border width and color, rules that match better than others (#id instead of .class) override previous definitions. `CartoCSS` acts the same way normally for the sake of familiarity and organization, but Mapnik itself is more powerful.
Layers in Mapnik can have multiple [borders](http://trac.mapnik.org/wiki/LineSymbolizer) and multiple copies of other attributes. This ability is useful in drawing line outlines, like in the case of road borders or 'glow' effects around coasts. `CartoCSS` makes this accessible by allowing attachments to styles:
This brings us to another _incompatibility_: `line-inline` and `line-outline` have been removed from the language, because attachments are capable of the same trick.
While attachments allow creating implicit "layers" with the same data, using **instances** allows you to create multiple symbolizers in the same style/layer:
#roads {
casing/line-width: 6;
casing/line-color: #333;
line-width: 4;
line-color: #666;
}
This makes Mapnik first draw the line of color #333 with a width of 6, and then immediately afterwards, it draws the same line again with width 4 and color #666. Contrast that to attachments: Mapnik would first draw all casings before proceeding to the actual lines.
Instead of the name attribute of the [TextSymbolizer](http://trac.mapnik.org/wiki/TextSymbolizer) and [ShieldSymbolizer](http://trac.mapnik.org/wiki/ShieldSymbolizer) being a part of the selector, it is a property of a rule. Thus the evaluation is less complex and one can use expressions in names.
Rasters are supported in CartoCSS - it knows how to download `.vrt`, `.tiff`, and soon other raster formats, and the properties of the [RasterSymbolizer](http://trac.mapnik.org/wiki/RasterSymbolizer) are exposed in the language.
The [BuildingSymbolizer](http://trac.mapnik.org/wiki/BuildingSymbolizer) is also supported in `CartoCSS`. The code stores symbolizer types and properties in a JSON file (in `tree/reference.json`), so new Mapnik features can be quickly implemented here.
CartoCSS inherits from its basis in [less.js](http://lesscss.org/) some new features in CSS. One can define variables in stylesheets, and use expressions to modify them.
By defining multiple fonts in a `text-face-name` definition, you create [FontSets](http://trac.mapnik.org/wiki/FontSet) in `CartoCSS`. These are useful for supporting multiple character sets and fallback fonts for distributed styles.