Tcl package, DTD validation extension to [tDOM]. Usage example contributed by [RS]:

 package require tnc ;# implies tdom

 #---------------------------------------- modified from expat man page
 proc externalEntityRefHandler {base systemId publicId} {
    if {![regexp {^[/a-zA-Z]+:/} $systemId]}  {
        regsub {^[/a-zA-Z]+:} $base {} base
        set basedir [file dirname $base]
        set systemId "[set basedir]/[set systemId]"
    }
    regsub {^[/a-zA-Z]+:} $systemId "" systemId
    set fd [open $systemId]
    list channel $systemId $fd
 }

 foreach file [glob $argv] {
    if [file readable $file] {
	set parser [expat -externalentitycommand externalEntityRefHandler\
			-baseurl "file://[file join [pwd] $file]" \
			-paramentityparsing notstandalone ]
	tnc $parser enable
	catch {$parser parsefile $file} res
	if {$res==""} {set res ok}
	puts $file:$res
    }
 }