The xsl:param element is used to declare them:

<xsl:param name="param-name-1"/>
<xsl:param name="param-name-2">default value</xsl:param>

These must occur at the top level.

A parameter can be set by the using application but the stylesheet can define defaults by:

• The children of xsl:param.

• A 'select' attribute.

So they typically have simple values.

Example:

<xsl:param name="background-color">white</xsl:param>