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?

<option selected="selected">Ham Burger</option> 

or

<input type="button" disabled="disabled" /> 

Thanks in advance!

Answer 1

In SGML, an attribute may be minimized so that its value alone is short for both the name and the value, with the only possible value for the attribute in this case obviously being the attribute's own name. HTML uses this for boolean attributes, where the presence or absence of the attribute is what's meaningful, and its value is irrelevant. But in XML, minimized attributes were disallowed, so we wound up with the awkwardness that is selected="selected" when XHTML came into vogue. If you're writing HTML rather than XHTML, you can just write selected.

Answer 2

The exact definition is:

Some attributes play the role of boolean variables (e.g., the selected attribute for the OPTION element). Their appearance in the start tag of an element implies that the value of the attribute is "true". Their absence implies a value of "false".

Also:

Boolean attributes may legally take a single value: the name of the attribute itself [...] In HTML, boolean attributes may appear in minimized form

Basically, this implies that there are only two possible statuses for boolean attributes, true and false, but there isn't a not set status.