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

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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.

10 Answers
10

Strictly speaking, you should put something that makes sense – according to the spec here, the most correct version is:

<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).

Effectively, however, most 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>

See also this similar question on disabled="disabled".

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