Not using output

Without an xsl:output, the result serializes as xml, or as html when its root element is <html>, and a reader has to infer which from the shape of the first element. Declare the output in the stylesheet a transformation starts at.

Incorrect:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <html><body><xsl:value-of select="."/></body></html>
  </xsl:template>
</xsl:stylesheet>

Correct:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="html" encoding="UTF-8"/>
  <xsl:template match="/">
    <html><body><xsl:value-of select="."/></body></html>
  </xsl:template>
</xsl:stylesheet>

Only an entry stylesheet is checked: one with a template matching the document root, the template named xsl:initial-template, or a default-mode template taking the top element. An unnamed xsl:output anywhere in its import tree counts. A library module or an xsl:package is left alone.