Interesting stuff. One way to think of this may be in terms of OPs
initial example. Here's a take on it, which I present both as
explanation and for correction.

 <xsl:variable name="hw">
   <xsl:text>Hello</xsl:text>
   <xsl:text>World</xsl:text>
 </xsl:variable>
 <xsl:value-of select="$hw"/>

An XSLT processor should spit out the string "HelloWorld", but
whether it generates that string by 

* take text node `Hello`
  take text node `World`, stick it *after* `Hello`

OR

* take text node `World`
  take text node `Hello`, stick it *before* `World`
  
is entirely up to the processor.