When testing in W3C, we got some errors such as:

Line 42, Column 98: document type does not allow element "form" here; missing one of "object", "applet", "map", "iframe", "ins", "del" start-tag

…earch_form" action="/index.php/" method="get">

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>").

Line 155, Column 43: document type does not allow element "div" here; missing one of "object", "applet", "map", "iframe", "button", "ins", "del" start-tag<div class="clear"><!-- DON'T DELETE --></div>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>").

Why?

Because in the code, i do as below:

<span class="left"><form action="XXX" method="XXX">//Other codes</form></span><span class="right"><div class="clear"><!-- DON'T DELETE --></div></span>

But that, put a block-level element (such as "<p>" or "<table>") inside an inline element (such as "<a>", "<span>", or "<font>") is not allowed in W3C, the form element must be contained within only certain elements, but many more types of elements can be contained within the form element. For example: <p><form></form></p> that will not validate. However, this will: <form><p></p></form>