Index: contrib/libxo/libxo/libxo.c
===================================================================
--- contrib/libxo/libxo/libxo.c
+++ contrib/libxo/libxo/libxo.c
@@ -6929,7 +6929,9 @@
 
     switch (xo_style(xop)) {
     case XO_STYLE_XML:
-	rc = xo_printf(xop, "%*s<%s", xo_indent(xop), "", name);
+	if (! isalpha(name[0]) && name[0] != '_')
+		pre_nl = "_";
+	rc = xo_printf(xop, "%*s<%s%s", xo_indent(xop), "", pre_nl, name);
 
 	if (xop->xo_attrs.xb_curp != xop->xo_attrs.xb_bufp) {
 	    rc += xop->xo_attrs.xb_curp - xop->xo_attrs.xb_bufp;
@@ -7028,7 +7030,10 @@
     switch (xo_style(xop)) {
     case XO_STYLE_XML:
 	xo_depth_change(xop, name, -1, -1, XSS_CLOSE_CONTAINER, 0);
-	rc = xo_printf(xop, "%*s</%s>%s", xo_indent(xop), "", name, ppn);
+	if (! isalpha(name[0]) && name[0] != '_')
+		pre_nl = "_";
+	rc = xo_printf(xop, "%*s</%s%s>%s", xo_indent(xop), "", pre_nl, name,
+	    ppn);
 	break;
 
     case XO_STYLE_JSON: