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

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>_x000D_
    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="on">_x000D_
    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="yes">_x000D_
    <input name="checkbox_name" id="checkbox_id" type="checkbox" checked="checked">_x000D_
    <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.

I think this may help:

Secondly, you need to be aware that the checked attribute RETURNS a string "true", "false"
Why is this important? Because you need to use the correct Type. A string, not a boolean. This also important when parsing your checkbox.
$('mycheckbox')[0].checked = "true"

if($('mycheckbox')[0].checked === "true"){ //do something }

You also need to realize that the "checked" ATTRIBUTE is for setting the value of the checkbox initially. This doesn't do much once the element is rendered to the DOM. Picture this working when the webpage loads and is initially parsed.
I'll go with IE's preference on this one: <input type="checkbox" checked="checked"/>

Lastly, the main aspect of confusion for a checkbox is that the checkbox UI element is not the same as the element's property value. They do not correlate directly. If you work in .net, you'll discover that the user "checking" a checkbox never reflects the actual bool value passed to the controller. To set the UI, I use both $('mycheckbox').val(true); and $('mycheckbox').attr('checked', 'checked');

you want this i think: checked='checked'

1. checked
2. checked=""

3. checked="checked"

   are equivalent;

according to spec checkbox '----? checked = "checked" or "" (empty string) or empty Specifies that the element represents a selected control.---'

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 />

Recommendation

If you care about writing valid XHTML, use checked="checked", since <input checked> is invalid XHTML (but valid HTML) and other alternatives are less readable. Else, just use <input checked> as it is shorter.

It's pretty crazy town that the only way to make checked false is to omit any values. With Angular 1.x, you can do this:

  <input type="radio" ng-checked="false">

which is a lot more sane, if you need to make it unchecked.

Well, to use it i dont think matters (similar to disabled and readonly), personally i use checked="checked" but if you are trying to manipulate them with JavaScript, you use true/false

<input ... checked />
<input ... checked="checked" />

Those are equally valid. And in JavaScript:

input.checked = true;
input.setAttribute("checked");
input.setAttribute("checked","checked");