The attribute is set to "true" by appearing in the element's start tag:
selected (selected) #IMPLIED -- option is pre-selected --
In HTML, boolean attributes may appear in minimized form -- the attribute's value appears alone in the element's start tag. Thus, selected may be set by writing:
<OPTION selected="selected"> ...contents... </OPTION>
Authors should be aware that many user agents only recognize the minimized form of boolean attributes and not the full form.