There are <extended-link>
  <from>several implementations</from>
  <to href="https://github.com/docbook/xslTNG"
      title="XSLT 3.0"/>
  <to label="target" href="https://github.com/docbook/xslt20-stylesheets"
      title="XSLT 2.0"/>
  <to label="target" href="https://github.com/docbook/xslt10-stylesheets"
      title="XSLT 1.0"/>
  <to label="target" href="https://github.com/docbook/dsssl"
      title="DSSSL"/>
</extended-link> of stylesheets for DocBook.