Show-Hide element in Dreamweaver CS5

Question

Please can you tell me why when I validate a web page I get the following error when using the Show-Hide behaviour.  The error is only reported for the first use of the feature and no of the others.  I don't understand.
Line 239,         Column 235:     there is no attribute "onload"…HideLayers('drive','','hide')" onload="MM_showHideLayers('drive','','hide')" /…
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You have used the attribute named above in your document, but the       document type you are using does not support that attribute for this       element. This error is often caused by incorrect use of the "Strict"       document type with a document that uses frames (e.g. you must use       the "Transitional" document type to get the "target" attribute), or       by using vendor proprietary extensions such as "marginheight" (this       is usually fixed by using CSS to achieve the desired effect instead).
This error may also result if the element itself is not supported in       the document type you are using, as an undefined element will have no       supported attributes; in this case, see the element-undefined error       message for further information.
How to fix: check the spelling and case of the element and attribute,        (Remember XHTML is all lower-case) and/or        check that they are both allowed in the chosen document type, and/or       use CSS instead of this attribute. If you received this error when using the        <embed> element to incorporate flash media in a Web page, see the        FAQ item on valid flash.
Below is a copy of the relevant code -The code in bold is the line causing the error.  You will notice I have used it more than once, but I didn't get any other error messages.  Please help.
  <h3>Artist Drive and Palette</h3>
    <!--  this enlarges two images using show/hide and two different sized images -->
   <div id="drive"><img src="Images/Artist Drive large.jpg" alt="Artist Drive" width="300" height="195" title="Artist Drive" onmouseover="MM_showHideLayers('drive','','show')" onmouseout="MM_showHideLayers('drive','','hide')" onload="MM_showHideLayers('drive','','hide')" />
    </div>
    <img src="Images/Artist Drive.jpg" alt="Artist Drive" width="100" height="56"  class="fltlft" title="Artist Drive" onmouseover="MM_showHideLayers('drive','','show')" onmouseout="MM_showHideLayers('drive','','hide')" onload="MM_showHideLayers('drive','','hide')" />
    <div id="palette"><img src="Images/Artist Palette large.jpg" alt="Artist's palette" width="300" height="194" title="Artist's Palette" onmouseover="MM_showHideLayers('palette','','show')" onmouseout="MM_showHideLayers('palette','','hide')" onload="MM_showHideLayers('palette','','hide')" />
    </div>
    <img src="Images/Artist Palette.jpg" alt="Artist Palette" width="100" height="60" class="fltrt" title="Artist's Palette" onmouseover="MM_showHideLayers('palette','','show')" onmouseout="MM_showHideLayers('palette','','hide')" onload="MM_showHideLayers('palette','','hide')" />
    <p class="size">These formations are in Death Vally, California, USA. Artist's  Drive rises up to the top of an alluvial fan fed by a deep canyon cut into the  Black Mountains. Artist's Palette is on the face of the Black Mountains and is noted for having various colours of rock. These colours are caused by the  oxidation of different metals (red, pink and yellow is from iron salts, green is from decomposing tuff-derived mica, and manganese produces the purple). Called the Artist Drive Formation, the rock unit provides evidence for one of the Death Valley area's most violently explosive volcanic periods. The Miocene-aged formation is made up of cemented gravel, playa deposits, and much volcanic debris, perhaps 5,000 feet (1500 m) thick. Chemical weathering and hydrothermal alteration are also responsible for the variety of colours displayed in the Artist Drive Formation and nearby exposures of the Furnace Creek Formation.
    </p>
    <h3>Badlands</h3>
     <!--  this enlarges two images using show/hide and two different sized images -->
    <div id="badland"><img src="Images/The Badlands large.jpg" alt="The Badlands" width="300" height="183" onmouseover="MM_showHideLayers('badland','','show')" onmouseout="MM_showHideLayers('badland','','hide')" onload="MM_showHideLayers('badland','','hide')" />
    </div>
    <p><img src="Images/The Badlands.jpg" alt="The Badlands" width="100" height="61" class="fltlft" onmouseover="MM_showHideLayers('badland','','show')" onmouseout="MM_showHideLayers('badland','','hide')" onload="MM_showHideLayers('badland','','hide')" />
    </p>
    <div id="badland1"><img src="Images/The Badlands1 large.jpg" alt="The Badlands" width="300" height="132" onmouseover="MM_showHideLayers('badland1','','show')" onmouseout="MM_showHideLayers('badland1','','hide')" onload="MM_showHideLayers('badland1','','hide')" />
    </div>
    <img src="Images/The Badlands1.jpg" alt="The Badlands" width="100" height="44" class="fltrt" onmouseover="MM_showHideLayers('badland1','','show')" onmouseout="MM_showHideLayers('badland1','','hide')" onload="MM_showHideLayers('badland1','','hide')" />
    <p class="size">Badlands National Park, in southwest South Dakota preserves 242,756 acres (98,240 ha) of sharply eroded buttes, pinnacles, and spires blended with the largest protected mixed grass prairie in the United States. On-going rains, freezes and thaws have revealed millions of years of sediment that paint a colourful landscape on the prairies of south-western South Dakota. It contains the world's richest Oligocene  epoch fossil beds, dating 23 to 35 million years old.
    </p>

Answer 1

Sometimes DW's javascripts will throw errors like that, most of the time it can be ignored.
What doctype are you using (the first line of your html)?
If you have a "Strict" Doctype, did you try changing it to a "Transitional" version as the error report suggested?
Edit > Preferences > New Document > Default Document Type (DTD) > Change from Strict to Transitional of the same version.