As said here:
A bareword attribute is a violation of the XML spec. The XHTML spec says that the best way around such cases where HTML has a bareword attribute (selected, checked, multiple, etc.) is to use selected="selected", checked="checked", and so on. An SGML HTML parser (web browser) will ignore the value it's given, and an XML XHTML parser (good web browser) will acknowledge the value, and ignore it. A file with selected="selected" is valid XHTML, according to the W3C's validation program.