<xsl:import> element is a top-level element that serves to import the contents of one stylesheet into another stylesheet. Generally speaking, the contents of the imported stylesheet have a lower import precedence than that of the importing stylesheet. This is in contrast to
<xsl:include> where the contents of the included stylesheet have exactly the same precedence as the contents of the including stylesheet.
<xsl:import href=URI />
Specifies the URI of the stylesheet to import.
Top-level, must appear before any other child of
<xsl:transform> in the importing stylesheet.
XSLT, section 2.6.2.
Mostly supported with a few issues with top level variables and parameters as of Mozilla 1.0.