see https://bugs.gentoo.org/944765
--- a/src/xml_validate.c
+++ b/src/xml_validate.c
@@ -28,6 +28,7 @@ THE SOFTWARE.
 
 #include <config.h>
 
+#include <unistd.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -408,15 +409,31 @@ valMain(int argc, char **argv)
         for (i=start; i<argc; i++)
         {
             int failed = 0;
+            int is_stdin = argv[i][0] == '-' && argv[i][1] == 0;
+
             if (ops.embed) options |= XML_PARSE_DTDVALID;
 
             if (!reader)
             {
-                reader = xmlReaderForFile(argv[i], NULL, options);
+                if (is_stdin)
+                {
+                    reader = xmlReaderForFd(STDIN_FILENO, argv[i], NULL, options);
+                }
+                else
+                {
+                    reader = xmlReaderForFile(argv[i], NULL, options);
+                }
             }
             else
             {
-                failed = xmlReaderNewFile(reader, argv[i], NULL, options);
+                if (is_stdin)
+                {
+                    failed = xmlReaderNewFd(reader, STDIN_FILENO, argv[i], NULL, options);
+                }
+                else
+                {
+                    failed = xmlReaderNewFile(reader, argv[i], NULL, options);
+                }
             }
 
             errorInfo.xmlReader = reader;