<div id="menu_droite">       
           <div class="element_menu_droite">
               <h4>Accès Membres</h4>
               <ul>
                 <FORM
                              <INPUT TYPE="text" NAME="login" SIZE="15" MAXLENGTH="40" VALUE="Login">
                              <INPUT TYPE="password" NAME="pass" SIZE="15" MAXLENGTH="40" VALUE="Password">
                                  <INPUT TYPE="submit" VALUE="Connexion">
                             </FORM>
               </ul>
           </div>
       
           <div class="element_menu_droite">
               <h4>Publicité</h4>
               <ul>
                                        <script type="text/javascript">
                                         <!--
                                                google_ad_client = "pub-2935266649149181";
                                                google_ad_width = 125;
                                                google_ad_height = 125;
                                                google_ad_format = "125x125_as";
                                                google_ad_type = "text_image";
                                                google_ad_channel ="";
                                                google_color_border = "2C485C";
                                                google_color_bg = "F4F9FD";
                                                google_color_link = "2C485C";
                                                google_color_url = "008000";
                                                google_color_text = "000000";
                                         //-->
                                        </script>
                                        <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"></script>
               </ul>
           </div>       
</div>

<form action="acces.php" />
        <input type="text" name="login" size="15" maxlength="40" value="Login" />
        <input type="password" name="pass" size="15" maxlength="40" value="Password" />
        <input type="submit" value="Connexion" />
</form>

# Error Line 64 column 14: end tag for element "form" which is not open.

</form>

The Validator found an end tag for the above element, but that element is not currently open. This is often caused by a leftover end tag from an element that was removed during editing, or by an implicitly closed element (if you have an error related to an element being used where it is not allowed, this is almost certainly the case). In the latter case this error will disappear as soon as you fix the original problem.

If this error occured in a script section of your document, you should probably read this FAQ entry.

<form  action="acces.php"
        <input type="text" name="login" size="15" maxlength="40" value="Login" />
        <input type="password" name="pass" size="15" maxlength="40" value="Password" />
        <input type="submit" value="Connexion" />
</form>

# Error Line 61 column 9: unclosed start-tag requires SHORTTAG YES.

<input type="text" name="login" size="15" maxlength="40" value="Login".

&#9993;
# Error Line 61 column 81: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag.

...e="15" maxlength="40" value="Login" />

The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.

One possible cause for this message is that you have attempted to put a block-level element (such as "<p>" or "<table>") inside an inline element (such as "<a>", "<span>", or "<font>").

&#9993;
# Error Line 62 column 87: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag.

...15" maxlength="40" value="Password" />

&#9993;
# Error Line 63 column 46: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag.

<input type="submit" value="Connexion" />

<nom arg="valeur"> contenu </nom>
et <nom arg="valeur" />
(en aucun cas un mélange des deux)