It can and has been able to for a good while.
On 2 Jun 2014, at 07:06, Andrew Pennebaker wrote:
> xmllint would play much more nicely with other Unix tools if XML could be piped into stdin when needed. Could the next version add this feature?
$ xmllint --version
xmllint: using libxml version 20627
compiled with: Threads Tree Output Push Reader Patterns Writer SAXv1 FTP HTTP DTDValid HTML Legacy C14N Catalog XPath XPointer XInclude Iconv ISO8859X Unicode Regexps Automata Expr Schemas Schematron Modules Debug Zlib
$ echo '<hello><there/></hello>' | xmllint --format -
<?xml version="1.0"?>
<hello>
<there/>
</hello>
--
Alex Bligh