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document type does not allow element "li" here; missing one of "ul", "ol" start-tag . 
                        <li><a href="index.html">Strona główna</a></li>
 
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>").
[HTML] pobierz, plaintext