HTML - Why boolean attributes do not have boolean value?

Question

I noticed that some elements have attributes which are boolean. I wonder why the values are not true or false? or 1 and 0? Are there any reason behind why they are like this

Answer 1

the HTML standard (Not the XHTML) is to have simply selected instead of selected="selected"

See here: http://www.w3.org/TR/html4/interact/forms.html#adef-selected

When XHTML was created to allow a a better integration with XML in HTML, (see http://www.w3.org/MarkUp/2004/xhtml-faq#need), the parts that do not fit to the XML-like structure requirements of HTML were corrected. So wordings like selected got transformed into selected="selected" to fit the standard