The "Documentation Development Tips & Tricks" page (http://www.gentoo.org/proj/en/gdp/doc/doc-tipsntricks.xml) includes instructions to use xmllint and xsltproc, but does not specify which packages provide these programs. "emerge --search xmllint" and "emerge --search xsltproc" won't return any results, so users not already familar with these programs must search for "xml" and "xslt", respectively, then guess that libxml2 and libxslt are what is needed. One solution is to modify the first section, "Setting up your local environment", to include a statement that libxml2 and libxslt must be present on the system.
Created attachment 92355 [details, diff] inline-package-names.patch Another solution is provided by this attachment, which mentions the package providing xmllint/libxslt inline, immediately after the first reference to it.
Good point. Fixed in CVS, thanks for reporting and for the patch.