Acceptable values for vertical-align

Could someone kindly clarify the acceptable values for 
"vertical-align".  In DocBook XSL (Stayton), it suggests (top, middle, 
bottom).  But in the xsl 1.67.2 distribution, the values of (sub, 
super) are used:

fo/footnote.xsl:        <xsl:attribute 
name="vertical-align">super</xsl:attribute>
fo/inline.xsl:        <xsl:attribute 
name="vertical-align">super</xsl:attribute>
fo/inline.xsl:        <xsl:attribute 
name="vertical-align">sub</xsl:attribute>
fo/xref.xsl:        <xsl:attribute 
name="vertical-align">super</xsl:attribute>

*Can a continuous variable be used, such as % ?

Cheers,        Tristan
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Tristan J. Fiedler
Postdoctoral Fellow - Stein Lab
Cold Spring Harbor Laboratory