(apologies if this was reported before, I thought we discussed this, but I could only find a similar issue in Bug 24109)

I checked the current Working Draft, both internal and public, and the following is considered not guaranteed streamable:

<xsl:template match="author" mode="streamable" as="xs:string">
   <xsl:sequence select="name" />
</xsl:template>

I think we should take the declared type into account with streamability analysis. We already have such rules elsewhere, as from bug 24109, it seems too restrictive to not allow such a construct, especially because I think (but can't claim) that people tend to use xsl:sequence over xsl:value-of nowadays.