I think according to W3 spec, you're supposed to do
<input type="checkbox" checked="checked" />
And
selected="selected"
But, most browsers will accept it you just write "CHECKED" and don't give it a value. So, what if you do include the attribute, are there any values that would be (consistently) considered false?
Actually, the HTML 4.01 spec says that these attributes do not require values. I haven't personally encountered a situation where providing a value rendered these controls as unselected.
Here are the respective links to the spec document for selected and checked.
Edit: Firebug renders the checkbox as checked regardless of any values I put in quotes for the checked attribute (including just typing "checked" with no values whatsoever), and IE 8's Developer Tools forces checked="checked". I'm not sure if any similar tools exist for other browsers that might alter the rendered state of a checkbox, however.
The empty string is false as a rule.
Apparently the empty string is not respected as empty in all browsers and the presence of the checked attribute is taken to mean checked. So the entire attribute must either be present or omitted.
The checked and selected attributes are allowed only two values, which are a copy of the attribute name and (from HTML 5 onwards) an empty string. Giving any other value is an error.
If you don't want to set the attribute, then the entire attribute must be omitted.
Note that in HTML 4 you may omit everything except the value. HTML 5 changed this to omit everything except the name (which makes no practical difference).
Thus, the complete (aside from variations in cAsE) set of valid representations of the attribute are:
<input ... checked="checked"> <!-- All versions of HTML / XHTML -->
<input ... checked > <!-- Only HTML 4.01 and earlier -->
<input ... checked > <!-- Only HTML 5 and later -->
<input ... checked="" > <!-- Only HTML 5 and later -->
Documents served as text/html (HTML or XHTML) will be fed through a tag soup parser, and the presence of a checked attribute (with any value) will be treated as "This element should be checked". Thus, while invalid, checked="true", checked="yes", and checked="false" will all trigger the checked state.
I've not had any inclination to find out what error recovery mechanisms are in place for XML parsing mode should a different value be given to the attribute, but I would expect that the legacy of HTML and/or simple error recovery would treat it in the same way: If the attribute is there then the element is checked.
(And all the above applies equally to selected as it does to checked.)
No value is considered false, only the absence of the attribute. There are plenty of invalid values though, and some implementations might consider certain invalid values as false.
HTML5 spec
http://www.w3.org/TR/html5/forms.html#attr-input-checked :
The disabled content attribute is a boolean attribute.
http://www.w3.org/TR/html5/infrastructure.html#boolean-attributes :
The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.
If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.
Conclusion
The following are valid, equivalent and true:
<input type="checkbox" checked />
<input type="checkbox" checked="" />
<input type="checkbox" checked="checked" />
<input type="checkbox" checked="ChEcKeD" />
The following are invalid:
<input type="checkbox" checked="0" />
<input type="checkbox" checked="1" />
<input type="checkbox" checked="false" />
<input type="checkbox" checked="true" />
The absence of the attribute is the only valid syntax for false:
<input type="checkbox" />
Recommendation
If you care about writing valid XHTML, use checked="checked", since <input checked> is invalid and other alternatives are less readable. Else, just use <input checked> as it is shorter.
Source: Stackoverflow.com