Query About Enabling / Disabling Form Inputs

https://forum.jquery.com/topic/query-about-enabling-disabling-form-inputs

HTML

<div id="book_form_section_1" class="active">
        <fieldset>
            <legend>About You</legend>
            <div>
                <label for="title">Title</label>
                <select name="title" id="title" class="input">
                    <option value="Mr">Mr</option>
                    <option value="Miss">Miss</option>
                    <option value="Ms">Ms</option>
                    <option value="Mrs">Mrs</option>
                </select>
            </div>
            <div>
                <label for="first_name">First Name</label>
                <input type="text" name="first_name" class="input required" required />
            </div>
            <div>
                <label for="last_name">Last Name</label>
                <input type="text" name="last_name" class="input"/>
            </div>
            <div>
                <label for="contact_number">Contact Number</label>
                <input type="text" name="contact_number" required  class="input required" required/>
            </div>
            <div>
                <label for="email">E-mail</label>
                <input type="text" name="email" class="input"/>
            </div>
            <div>
                <label for="confirm_email">Confirm E-mail</label>
                <input type="text" name="confirm_email" class="input"/>
            </div>
        </fieldset>
        </div>

CSS

body{font-size:0.9em;}
div{margin:3px;}
.required{border:1px solid #ff0000;}
#book_form_section_1{padding-bottom:10px; margin-bottom:10px;border-bottom:1px solid #444;}
.inactive{
    /* IE 8 */
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=20)";
    /* IE 5-7 */
    filter: alpha(opacity=20);
    /* Netscape */
    -moz-opacity: 0.2;
    /* Safari 1.x */
    -khtml-opacity: 0.2;
    /* Good browsers */
    opacity: 0.2;
}

JavaScript

function checkRequiredFields() {
        $('#book_form_section_1').addClass('inactive');
        $('input,select', '#book_form_section_1').attr('disabled', 'disabled');
}

checkRequiredFields();