--- a/src/java.desktop/share/classes/com/sun/imageio/plugins/gif/GIFImageMetadataFormat.java	Wed Mar 21 12:08:16 2018 +0530
+++ b/src/java.desktop/share/classes/com/sun/imageio/plugins/gif/GIFImageMetadataFormat.java	Thu Mar 22 11:41:15 2018 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2004, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -119,10 +119,10 @@
                      "1", "65535", true, true);
         addAttribute("PlainTextExtension", "characterCellWidth",
                      DATATYPE_INTEGER, true, null,
-                     "1", "65535", true, true);
+                     "1", "255", true, true);
         addAttribute("PlainTextExtension", "characterCellHeight",
                      DATATYPE_INTEGER, true, null,
-                     "1", "65535", true, true);
+                     "1", "255", true, true);
         addAttribute("PlainTextExtension", "textForegroundColor",
                      DATATYPE_INTEGER, true, null,
                      "0", "255", true, true);

--- a/src/java.desktop/share/classes/com/sun/imageio/plugins/gif/GIFWritableImageMetadata.java	Wed Mar 21 12:08:16 2018 +0530
+++ b/src/java.desktop/share/classes/com/sun/imageio/plugins/gif/GIFWritableImageMetadata.java	Thu Mar 22 11:41:15 2018 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2008, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -203,15 +203,18 @@
                                                  -1, true,
                                                  true, 1, 65535);
 
+                // As per the specification (89a), character cell width
+                // and character cell height occupy one byte each
+                // in the Plain Text Extension block.
                 characterCellWidth = getIntAttribute(node,
                                                      "characterCellWidth",
                                                      -1, true,
-                                                     true, 1, 65535);
+                                                     true, 1, 255);
 
                 characterCellHeight = getIntAttribute(node,
                                                       "characterCellHeight",
                                                       -1, true,
-                                                      true, 1, 65535);
+                                                      true, 1, 255);
 
                 textForegroundColor = getIntAttribute(node,
                                                       "textForegroundColor",

--- a/src/java.desktop/share/classes/javax/imageio/metadata/doc-files/gif_metadata.html	Wed Mar 21 12:08:16 2018 +0530
+++ b/src/java.desktop/share/classes/javax/imageio/metadata/doc-files/gif_metadata.html	Thu Mar 22 11:41:15 2018 +0530
@@ -5,7 +5,7 @@
   <title>GIF Metadata Format Specification</title>
 </head>
 <!--
-Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
+Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
 DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 
 This code is free software; you can redistribute it and/or modify it
@@ -291,13 +291,13 @@
         &lt;!-- The width of a character cell --&gt;
         &lt;!-- Data type: Integer --&gt;
         &lt;!-- Min value: 1 (inclusive) --&gt;
-        &lt;!-- Max value: 65535 (inclusive) --&gt;
+        &lt;!-- Max value: 255 (inclusive) --&gt;
       &lt;!ATTLIST "PlainTextExtension" "characterCellHeight" #CDATA
          #REQUIRED&gt;
         &lt;!-- The height of a character cell --&gt;
         &lt;!-- Data type: Integer --&gt;
         &lt;!-- Min value: 1 (inclusive) --&gt;
-        &lt;!-- Max value: 65535 (inclusive) --&gt;
+        &lt;!-- Max value: 255 (inclusive) --&gt;
       &lt;!ATTLIST "PlainTextExtension" "textForegroundColor" #CDATA
          #REQUIRED&gt;
         &lt;!-- The text foreground color index --&gt;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/javax/imageio/plugins/gif/GIFCharCellDimensionTest.java	Thu Mar 22 11:41:15 2018 +0530
@@ -0,0 +1,160 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 6686236
+ * @summary Checks whether GIF native metadata format returns appropriate max
+ *          value that could be set for characterCellWidth and
+ *          characterCellHeight attributes of PlainTextExtension node. Besides,
+ *          the test also checks whether IIOInvalidTreeException is thrown when
+ *          incorrect value is set on these two attributes.
+ * @run main GIFCharCellDimensionTest
+ */
+import javax.imageio.ImageIO;
+import javax.imageio.ImageWriter;
+import javax.imageio.ImageTypeSpecifier;
+import javax.imageio.metadata.IIOInvalidTreeException;
+import javax.imageio.metadata.IIOMetadata;
+import javax.imageio.metadata.IIOMetadataNode;
+import javax.imageio.metadata.IIOMetadataFormat;
+import java.util.Iterator;
+import java.awt.image.BufferedImage;
+import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR;
+
+public class GIFCharCellDimensionTest {
+    // GIF image metadata and native metadata format.
+    private static IIOMetadata imageMetadata = null;
+    private static IIOMetadataFormat metadataFormat = null;
+    private static String formatName = null;
+
+    private static void initializeTest() {
+        // Initialize the test case by querying GIF image writer for default
+        // image metadata and native metadata format.
+        ImageWriter gifWriter = null;
+        Iterator<ImageWriter> iterGifWriter = null;
+        BufferedImage bufImage = null;
+        ImageTypeSpecifier imageType = null;
+
+        try {
+            iterGifWriter = ImageIO.getImageWritersBySuffix("GIF");
+            if (iterGifWriter.hasNext()) {
+                gifWriter = iterGifWriter.next();
+                bufImage = new BufferedImage(32, 32, TYPE_4BYTE_ABGR);
+
+                // Get GIF image metadata
+                imageMetadata = gifWriter.getDefaultImageMetadata(
+                        ImageTypeSpecifier.createFromRenderedImage(bufImage),
+                        gifWriter.getDefaultWriteParam());
+                if (imageMetadata == null) {
+                    reportException("Test Failed. Could not get image" +
+                            " metadata.");
+                }
+
+                // Get GIF native metadata format.
+                formatName = imageMetadata.getNativeMetadataFormatName();
+                metadataFormat = imageMetadata.getMetadataFormat(formatName);
+                if (metadataFormat == null) {
+                    reportException("Test Failed. Could not get native" +
+                            " metadata format.");
+                }
+            } else {
+                reportException("Test Failed. No GIF image writer found.");
+            }
+        } finally {
+            gifWriter.dispose();
+        }
+    }
+
+    private static IIOMetadataNode createPlainTextExtensionNode(String value) {
+        // Create a PlainTextExtensionNode with required values
+        IIOMetadataNode rootNode = null;
+
+        if (imageMetadata != null && formatName != null) {
+            IIOMetadataNode plainTextNode = null;
+
+            rootNode = new IIOMetadataNode(formatName);
+            plainTextNode = new IIOMetadataNode("PlainTextExtension");
+            plainTextNode.setAttribute("textGridLeft", "0");
+            plainTextNode.setAttribute("textGridTop", "0");
+            plainTextNode.setAttribute("textGridWidth", "32");
+            plainTextNode.setAttribute("textGridHeight", "32");
+            // Set required values for cell width and cell height
+            plainTextNode.setAttribute("characterCellWidth", value);
+            plainTextNode.setAttribute("characterCellHeight", value);
+            plainTextNode.setAttribute("textForegroundColor", "0");
+            plainTextNode.setAttribute("textBackgroundColor", "1");
+            rootNode.appendChild(plainTextNode);
+        } else {
+            reportException("Test Failed. Un-initialized image metadata.");
+        }
+
+        return rootNode;
+    }
+
+    private static void testCharacterCellDimensions() {
+        if (imageMetadata != null && metadataFormat != null) {
+            // Get max permissible value that could be set on characterCellWidth
+            // and characterCellHeight attributes of plain text node.
+            String cellWidth = metadataFormat.getAttributeMaxValue(
+                    "PlainTextExtension",
+                    "characterCellWidth");
+            String cellHeight = metadataFormat.getAttributeMaxValue(
+                    "PlainTextExtension",
+                    "characterCellHeight");
+
+            // Test fails if the max permissible value is larger than 255
+            int maxCharCellWidth = Integer.parseInt(cellWidth);
+            int maxCharCellHeight = Integer.parseInt(cellHeight);
+            if (maxCharCellWidth > 255 || maxCharCellHeight > 255) {
+                reportException("Test Failed. Invalid max range for" +
+                        " character cell width or character cell height.");
+            }
+
+            try {
+                // Merge a plain text node with incorrect value set on
+                // characterCellWidth and characterCellHeight attributes.
+                IIOMetadataNode root = createPlainTextExtensionNode("256");
+                imageMetadata.setFromTree(formatName, root);
+            } catch (IIOInvalidTreeException exception) {
+                // No-op. Expected exception was thrown.
+            }
+        } else {
+            reportException("Test Failed. Un-initialized image metadata or" +
+                    " metadata format.");
+        }
+    }
+
+    private static void reportException(String message) {
+        // Report an exception with the required message.
+        throw new RuntimeException(message);
+    }
+
+    public static void main(String[] args) {
+        // Initialize the test
+        initializeTest();
+
+        // Test plain text extension node.
+        testCharacterCellDimensions();
+    }
+}
\ No newline at end of file