15. url() is XML catalog aware
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## 15. url() is XML catalog aware

The URI specified using the standard `url()` pseudo-function may be resolved using XML catalogs.

For example, this feature can be used to customize the DocBook CSS style sheet bundled with XXE:

    @import url(xxe-config:docbook/css/docbook.css);
          .
          .
       my customization here
          .

| Note that `@import "xxe-config:docbook/css/docbook.css";` works fine too. That is, in the case of `@import`, the `url()` pseudo-function is not strictly necessary for the XML catalogs to be used to resolve the URI.

This works because the XML catalog bundled with XXE, ``XXE_install_dir`/addon/config/catalog.xml`, contains the following rule:

    <rewriteURI uriStartString="xxe-config:" rewritePrefix="." />

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