In XHTML 1.0 Strict the tag is not Permitted

hi what do I wrong ? or what do sharepoint designer 2013 wrong ?

In SharePoint designer 2013 I make a aspx site, inside the <form id="form1" runat="server"></form>
I place a Data View (WebPartPages:DataFormWebPart), then some red underlines is placed in some of the code, and when I place the cursor over the red underline a box is pop up and with the message: In XHTML 1.0 Strict the tag <WebPartPages:DataformWebpart> is not Permitted.

See below