The DocBook stylesheet html/docbook.xsl produces a single HTML file from your DocBook XML input file. The stylesheet itself does not name the file, so you have to supply the output filename. You can do this with the specific output option of your XSL processor, or you can simply redirect the output of the processor to a file.

xsltproc  --output  myfile.html  docbook.xsl  myfile.xml
or
xsltproc  docbook.xsl  myfile.xml  >  myfile.html

If you want the output to appear in another directory, then use a relative or absolute pathname instead of just the filename. For example

xsltproc  --output  /usr/apache/htdocs/myfile.html  docbook.xsl  myfile.xml

Several other options for single-file output are described in the section “Single file options with onechunk”.