Index: configure
==================================================================
--- configure
+++ configure
@@ -1376,11 +1376,11 @@
 
 Optional Features:
   --disable-option-checking  ignore unrecognized --enable/--with options
   --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
   --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]
-  --enable-html5          build with HTML5 parsing support (default: no)
+  --enable-html5          build with HTML5 parsing support (default: off)
   --enable-threads        build with threads
   --enable-shared         build and link with shared libraries (default: on)
   --enable-stubs          build and link with stub libraries. Always true for
                           shared builds (default: on)
   --enable-64bit          enable 64bit support (default: off)
@@ -5542,22 +5542,34 @@
         tcl_ok=$enableval
     else
         tcl_ok=no
     fi
     HTML5_LIBS=""
+    HTML5_INCLUDES=""
+    if test "$tcl_ok" = "yes" ; then
+        # Check if pkg-config is available
+        PKGCONFIG=no
+        pkg-config --version > /dev/null 2>&1 && PKGCONFIG=yes
+        if test "$PKGCONFIG" = no; then
+            tcl_ok=no
+	    as_fn_error $? "**** Cannot find pkg-config needed for --enable-html5." "$LINENO" 5
+	    as_fn_error $? "**** Despite configure request HTML5 parser _NOT_ enabled." "$LINENO" 5
+        fi
+    fi
     if test "$tcl_ok" = "yes" ; then
         HAVEGUMBO=`pkg-config --exists gumbo && echo "1"`
         if test "$HAVEGUMBO" = "1" ; then
             { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
             $as_echo "#define TDOM_HAVE_GUMBO 1" >>confdefs.h
 
             if test "${TEA_PLATFORM}" = "windows" ; then
-                HTML5_LIBS="-Wl,-Bstatic `pkg-config --static --cflags --libs gumbo` -Wl,-Bdynamic"
+                HTML5_LIBS="-Wl,-Bstatic `pkg-config --static --libs gumbo` -Wl,-Bdynamic"
             else
-                HTML5_LIBS="`pkg-config --cflags --libs gumbo`"
+                HTML5_LIBS="`pkg-config --libs gumbo`"
             fi
+            HTML5_INCLUDES="`pkg-config --cflags gumbo`"
         else
             as_fn_error $? "The required lib gumbo not found" "$LINENO" 5
         fi
     else
         { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
@@ -5633,11 +5645,11 @@
 	PKG_HEADERS="$PKG_HEADERS $i"
     done
 
 
 
-    vars="-I${srcdir}/generic ${AOL_INCLUDES}"
+    vars="-I${srcdir}/generic ${AOL_INCLUDES} ${HTML5_INCLUDES}"
     for i in $vars; do
 	PKG_INCLUDES="$PKG_INCLUDES $i"
     done
 
 

Index: configure.in
==================================================================
--- configure.in
+++ configure.in
@@ -108,11 +108,11 @@
                  generic/tdominit.c  \
                  generic/tclexpat.c  \
                  generic/tclpull.c   \
                  generic/tdomStubInit.c])
 TEA_ADD_HEADERS([generic/tdom.h])
-TEA_ADD_INCLUDES([-I${srcdir}/generic ${AOL_INCLUDES}])
+TEA_ADD_INCLUDES([-I${srcdir}/generic ${AOL_INCLUDES} ${HTML5_INCLUDES}])
 TEA_ADD_LIBS([${AOL_LIBS} ${HTML5_LIBS}])
 TEA_ADD_CFLAGS([])
 TEA_ADD_STUB_SOURCES([generic/tdomStubLib.c])
 TEA_ADD_TCL_SOURCES([lib/tdom.tcl])
 

Index: tdom.m4
==================================================================
--- tdom.m4
+++ tdom.m4
@@ -226,30 +226,41 @@
 
 AC_DEFUN(TDOM_ENABLE_HTML5, [
     AC_MSG_CHECKING([whether to enable support for HTML5 parsing (using gumbo)])
     AC_ARG_ENABLE(html5,
         AC_HELP_STRING([--enable-html5],
-            [build with HTML5 parsing support (default: no)]),
+            [build with HTML5 parsing support (default: off)]),
         [tcl_ok=$enableval], [tcl_ok=no])
 
     if test "${enable_html5+set}" = set; then
         enableval="$enable_html5"
         tcl_ok=$enableval
     else
         tcl_ok=no
     fi
     HTML5_LIBS=""
+    HTML5_INCLUDES=""
+    if test "$tcl_ok" = "yes" ; then
+        # Check if pkg-config is available
+        PKGCONFIG=no
+        pkg-config --version > /dev/null 2>&1 && PKGCONFIG=yes
+        if test "$PKGCONFIG" = no; then
+            tcl_ok=no
+	    AC_MSG_ERROR([cannot find pkg-config needed for --enable-html5.])
+        fi
+    fi
     if test "$tcl_ok" = "yes" ; then
         HAVEGUMBO=`pkg-config --exists gumbo && echo "1"`
         if test "$HAVEGUMBO" = "1" ; then
             AC_MSG_RESULT([yes])
             AC_DEFINE(TDOM_HAVE_GUMBO)
             if test "${TEA_PLATFORM}" = "windows" ; then
-                HTML5_LIBS="-Wl,-Bstatic `pkg-config --static --cflags --libs gumbo` -Wl,-Bdynamic"
+                HTML5_LIBS="-Wl,-Bstatic `pkg-config --static --libs gumbo` -Wl,-Bdynamic"
             else
-                HTML5_LIBS="`pkg-config --cflags --libs gumbo`"
+                HTML5_LIBS="`pkg-config --libs gumbo`"
             fi
+            HTML5_INCLUDES="`pkg-config --cflags gumbo`"
         else
             AC_MSG_ERROR([The required lib gumbo not found])
         fi
     else    
         AC_MSG_RESULT([no])