What's the proper value for a checked attribute of an HTML checkbox?

Question

We all know how to form a checkbox input in HTML:

<input name="checkbox_name" id="checkbox_id" type="checkbox">

What I don't know -- what's the technically correct value for a checked checkbox? I've seen these all work:

<input name="checkbox_name" id="checkbox_id" type="checkbox" checked>

    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="on">

    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="yes">

    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="checked">

    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="true">

Is the answer that it doesn't matter? I see no evidence for the answer marked as correct here from the spec itself:

Checkboxes (and radio buttons) are on/off switches that may be toggled by the user. A switch is "on" when the control element's checked attribute is set. When a form is submitted, only "on" checkbox controls can become successful. Several checkboxes in a form may share the same control name. Thus, for example, checkboxes allow users to select several values for the same property. The INPUT element is used to create a checkbox control.

What would a spec writer say is the correct answer? Please provide evidence-based answers.

Answer 1

Try doing something like this:

<input name=name id=id type=checkbox checked=checked>

For HTML, you can also use the empty attribute syntax, checked=" ", or even simply checked (for stricter XHTML, this is not supported).

Since, most of the browsers will support just about any value between the quotes. 

All of the following will be checked:

<input name=name id=id type=checkbox checked>

<input name=name id=id type=checkbox checked="">

<input name=name id=id type=checkbox checked="yes">

<input name=name id=id type=checkbox checked="blue">

<input name=name id=id type=checkbox checked="false">

And only the following will be unchecked:

<input name=name id=id type=checkbox>

Have a look at this document, for more information: disabled="disabled".