Why can't radio buttons be "readonly"?

Radio buttons would only need to be read-only if there are other options. If you don't have any other options, a checked radio button cannot be unchecked. If you have other options, you can prevent the user from changing the value merely by disabling the other options:

<input type="radio" name="foo" value="Y" checked>
<input type="radio" name="foo" value="N" disabled>

Fiddle: http://jsfiddle.net/qqVGu/

This is the trick you can go with.

<input type="radio" name="name" onclick="this.checked = false;" />

Try the attribute disabled, but I think the you won't get the value of the radio buttons. Or set images instead like:

<img src="itischecked.gif" alt="[x]" />radio button option

Best Regards.

A fairly simple option would be to create a javascript function called on the form's "onsubmit" event to enable the radiobutton back so that it's value is posted with the rest of the form.
It does not seem to be an omission on HTML specs, but a design choice (a logical one, IMHO), a radiobutton can't be readonly as a button can't be, if you don't want to use it, then disable it.

I'm using a JS plugin that styles checkbox/radio input elements and used the following jQuery to establish a 'readonly state' where the underlying value is still posted but the input element appears inaccessible to the user, which is I believe the intended reason we would use a readonly input attribute...

if ($(node).prop('readonly')) {
    $(node).parent('div').addClass('disabled'); // just styling, appears greyed out
    $(node).on('click', function (e) {
        e.preventDefault();
    });
}

Here is my solution (override) for Sencha ExtJS 7.2+ (checkbox and radio in a single override)

Ext.define('MyApp.override.field.Checkbox', {
    override: 'Ext.field.Checkbox',

    /**
     * OVERRIDE: to allow updateReadOnly to work propperly
     * @param {boolean} newValue
     *
     * To ensure the disabled state stays active if the field is still readOnly
     * we re - set the disabled state
     */
    updateDisabled(newValue) {
        this.callParent(arguments);

        if (!newValue && this.getReadOnly()) {
            this.inputElement.dom.disabled = true;
        }
    },

    /**
     * OVERRIDE: readonly for radiogroups and checkboxgroup do not work as other fields
     *     https://stackoverflow.com/questions/1953017/why-cant-radio-buttons-be-readonly
     *
     * @param {boolean} newValue
     *
     *  - use disabled on the field
     */
    updateReadOnly(value) {
        this.callParent(arguments);

        if (!this.getDisabled()) {
            this.inputElement.dom.disabled = value;
        }
    }
});

Disabled works on individual radio buttons (not the whole set). If you are using PHP and your circumstances are known on the server before loading the page in the browser you can use this very simple solution.

<php $chcky= (condition) ? 'checked' : 'disabled';?>
<php $chckn= (condition) ? 'checked' : 'disabled';?>
<input type="radio" name="var" value="Yes" <?php echo $chcky?>></input>
<input type="radio" name="var" value="No" <?php echo $chckn?>></input>

I use it on the page responding a form submission.

Extract from https://stackoverflow.com/a/71086058/18183749

If you can't use the 'disabled' attribut (as it erases the value's input at POST), and noticed that html attribut 'readonly' works only on textarea and some input(text, password, search, as far I've seen), and finally, if you don't want to bother with duplicating all your select, checkbox and radio with hidden input, you might find the following function or any of his inner logics to your liking :

addReadOnlyToFormElements = function (idElement) {
    // html readonly don't work on input of type checkbox and radio, neither on select. So, a safe trick is to disable the non-selected items
    $('#' + idElement + ' input[type="radio"]:not(:checked)').prop('disabled',true); 

    // and, on the selected ones, mimic readonly appearance
    $('#' + idElement + ' input[type="radio"]:checked').css('opacity','0.5');
}

And there's nothing easier than to remove these readonly

removeReadOnlyFromFormElements = function (idElement) {
    // Remove the disabled attribut on non-selected
    $('#' + idElement + ' input[type="radio"]:not(:checked)').prop('disabled',false); 
 
    // Remove readonly appearance on selected ones
    $('#' + idElement + ' input[type="radio"]:checked').css('opacity','');
}

What about capturing an "On_click()" event in JS and checking it like here?:

http://www.dynamicdrive.com/forums/showthread.php?t=33043