my post is being cleared whenever there is <iframe> in it

hi here is my problem I have a form

<div class="default-form" id="website_form" data-method="website">
    <div class="panel-heading panel-heading-transparent"> <strong>Website Information</strong> </div>
    <div class="panel-body">
        <form class="validate well" id="default_form" onsubmit="return false;" data-toastr-position="top-right" data-success="Sent! Thank you!" novalidate="novalidate">
            <fieldset> 
                <div class="row">
                    <div class="form-group">
                        <div class="col-md-12 col-sm-12"> <label>Title *</label> <input type="text" class="form-control required" value="{$set_data.DEF_TITLE}" name="d[DEF_TITLE]" id="DEF_TITLE"> </div>
                    </div>
                </div>
            </fieldset>
            <div class="row">
                <div class="form-group">
                    <div class="col-md-12 col-sm-12"> <button id="submit_form" data-fid="default_form" class="btn btn-3d btn-primary btn-xlg btn-block margin-top-30" type="submit">
                            UPDATE
                        </button> </div>
                </div>
            </div>
        </form>
    </div>
</div>

and the parameteres being sent is this

d[DEF_TITLE]    sample message <iframe></iframe>

and the source is this

DEF_TITLE%5D=sample+message+%3Ciframe%3E%3C%2Fiframe%3E

after I submit my form via ajax and now my code for my serverside (where the post will be directed) is this

function procedure()
    {
            $fs = $this->input->post('d');
            print_r('bbb');
            print_r($fs);
            exit;
    }

I recieve empyt in my post. But if I remove the <iframe> or <iframe></iframe> in the text that I will input in the form, the post will now have a value. How is this possible? I used php codeigniter only. Please help me for I really tried all that I can think of... Thanks...

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Additional Info for my ajax used:

var posts = $('#default_form').serialize();
    $.post(path + "/procedure/", posts, function(result) {
    var result = $.parseJSON(result);
});

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Additional Info seems like html tag makes the post disappeared only. I tried "test ", "test ", "test ", "test ", "test ", "test

", "test " and they all works well in post