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HTML 

              
                <div id="content">
		<div id='popup_content'>
			<h1>Custom select</h1>
			<fieldset>
				<legend>Simple Select</legend>
				<form id="form_language">
				<label for="language">My favorite language</label>
				<div class="select" >
					<div class='select_input' id="id_select_input" >
						<input type="text" id="language" name="language" readonly="readonly" class='select_text' placeholder="Select a language"/>
						<div class='select_icon'>
							<span class='caret'></span>
						</div>
					</div>
					<div class='select_box'>
						<div class='select_item' data-valeur="JavaScript">
							JavaScript
						</div>
						<div class='select_item'data-valeur="PHP">
							PHP
						</div>
						<div class='select_item' data-valeur="CSS">
							CSS
						</div>
						<div class='select_item' data-valeur="HTML">
							HTML
						</div>
						<div class='select_item' data-valeur="Python">
							Python
						</div>
						<div class='select_item' data-valeur="Ruby">
							Ruby
						</div>
						<div class='select_item' data-valeur="C#">
							C#
						</div>
						<div class='select_item' data-valeur="Java">
							Java
						</div>
						<div class='select_item' data-valeur="C/C++">
							C/C++
						</div>
						<div class='select_item' data-valeur="Bash">
							Bash
						</div>
					</div>
				</div>
				<input type="button" value="Display">
				</form>
			</fieldset>
			<fieldset>
			<legend>Select with clickable optgroup</legend>
				<form id="form_framework">
				<label for="language">My favorite Framework</label>
				<div class="select" >
					<div class='select_input' >
						<input type="text" id="framework" name="framework" readonly="readonly" class='select_text' placeholder="Select a framework"/>
						<div class='select_icon'>
							<span class='caret'></span>
						</div>
					</div>
					<div class='select_box'>
						<div class='select_group'>
							<div class='select_header'>
								PHP
								<div class='select_icon'>
									<span class='caret'></span>
								</div>
							</div>
							<div class='select_item' data-valeur="Symfony">
								Symfony
							</div>
							<div class='select_item' data-valeur="CakePHP">
								CakePHP
							</div>
							<div class='select_item' data-valeur="Laravel">
								Laravel
							</div>
							<div class='select_item' data-valeur="Zend Framework">
								Zend Framework
							</div>
							<div class='select_item' data-valeur="CodeIgniter">
								CodeIgniter
							</div>
						</div>
						<div class='select_group'>
							<div class='select_header'>
								JavaScript
								<div class='select_icon'>
									<span class='caret'></span>
								</div>
							</div>
							<div class='select_item' data-valeur="JQuery">
								JQuery
							</div>
							<div class='select_item' data-valeur="AngularJS">
								AngularJS
							</div>
							<div class='select_item' data-valeur="VanillaJS">
								VanillaJS
							</div>
							<div class='select_item'data-valeur="React">
								React
							</div>
						</div>

						<div class='select_group'>
							<div class='select_header'>
								Python
								<div class='select_icon'>
									<span class='caret'></span>
								</div>
							</div>
							<div class='select_item' data-valeur="Django">
								Django
							</div>
							<div class='select_item' data-valeur="Pyramid">
								Pyramid
							</div>
							<div class='select_item' data-valeur="TurboGears">
								TurboGears
							</div>
							
						</div>
					
						<div class='select_group'>
							<div class='select_header'>
								CSS
								<div class='select_icon'>
									<span class='caret'></span>
								</div>
							</div>
							<div class='select_item' data-valeur="Bootstrap">
								Bootstrap
							</div>
							<div class='select_item' data-valeur="Zurb Fundation">
								Zurb Fundation
							</div>
							<div class='select_item' data-valeur="Bulma">
								Bulma
							</div>
						</div>
						
					</div>
				</div>
				<input type="button" value="Display">
		</form>
		</fieldset>
		
		</div>
	</div>
		<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
!

CSS 

              
                /**
 * Custom select file style
 * @author Alicia Malacarne
 * 2017-2019
 * */
body{
	font-family:arial;
	font-size:18px;
}
 #content{
 	width:70%;
 	margin:0 auto;
 }
 fieldset{
 	margin:4% auto;
 	width:50%;
 	border-radius: 5px;
 	box-shadow: 1px 2px 2px 2px #696969;
 	border:none;
 	text-align: center;
 }
 legend{
 	padding: 0 5px;
 	font-weight: bold;
 	font-size:20px;
 	background-color: #fff;
 }
label, .select, input[type="button"] {
	max-width: 90%;
	margin: 3%;
}
.select .select_box {
	display: none;
	flex-direction: column;
	flex-wrap: nowrap;
	z-index: 99999;
	margin-top:0;
	position: absolute;
	border: 1px solid #1E90FF;
	box-shadow:2px 2px 2px #696969;
	
	overflow-x: hidden;
	overflow-y: visible;
}
.select .select_input {
	display: flex;
	flex-wrap:nowrap;
	width:100%;
	padding:0;
}

.select_input, input[type="button"]{
	height:44px;
	background-color: #D3D3D3;
	border: 1px solid #7F7F7F;
}

.select_icon{
	width:9%;
	min-width: 7px;
	display:flex;
	justify-content: center;
	align-items:center; 
	height:100%;
	text-align:center;
}
/*down arrow*/
.caret{
	border-top:7px solid #111111;
	border-left:7px solid transparent;
	border-right:7px solid transparent;
	border-bottom:7px solid transparent;
	display:inline-block;
}
.remove::after{
	content:"x";
	font-family: verdana;
	font-weight:bold;
	font-size:20px;
	color: #111111;
}

.select_item, .select_header {
	display:flex;
	align-items: center;
	justify-content: space-between;
	flex-wrap:nowrap;
 	width: 99%;
	height:44px;
	height:2.75em;
	padding: 0;
	padding-left: 1%;
	text-align: left;
}
.select_input *, input[type="button"], .select_header, .select_item {
	cursor: pointer;
}
.select_text {
	width: 90%;
	font-size:16px;
	text-align: left;
	flex:3;
	padding: 0;
	padding-left: 5px;
	border:none;
}
.select_input .select_text {
	color: #C0C0C0;
	background-color: #fff;
	height: 100%;
}

.select .select_box .select_group {
	width: 100%;
}
.select_item {
	background-color: #fff;
}
.select_group .select_item {
	display:none;
}
.select_header {
	background-color: #E5E8E8;
	border-bottom: 1px solid #ABB2B9;
	z-index:1000;
	font-weight: bold;
}

.select_item:nth-child(odd) {
	background-color: #DCDCDC;
}

.select_header:hover, .select_header[data-sslist='show'] {
	background-color: #7b76ab;
}
.select_item:hover{
	background-color:#B0C4DE;
}
/*fond invisible pour fermer la liste*/
#fade_transparant{
	display: none; /*--masqué par défaut--*/
	position: fixed; left: 0; top: 0;
	width: 100%; height: 100%;
	opacity: .0;
	z-index: 999;
}
/*bug IE6*/
*html #fade_transparant {
position: absolute;
}
*html .select {
position: absolute;
}


@media screen and (max-width:1024px){
	#content{
		width:100%;
	}
	.select_text{
		overflow: hidden;
	}
}
/*adapter les styles des ecrans tablettes et petits ordi*/
@media screen and (min-width:768px) and (max-width:979px){
	.select_text, .select_box .select_item, .select_header {
		font-size: 16px;
	}
	.select_input, .select_box .select_item, .select_header {
		height:40px;
		height:2.5rem;
	}
}
	
/*style ecrans smartphones et petites tabletts*/
@media screen and (max-width:767px){
	fieldset{
		width:70%;
	}
	.select_text, .select_box .select_item, .select_header {
		font-size: 14px;
	}
	.select_input, .select_item, .select_header {
		height:36px;
		height:2.25rem;
	}
}
/*smartphones*/
@media screen and (max-width:480px){
	fieldset{
		width:90%;
	}
	.select_box{
		max-height:90%;
		max-width:90%;
		margin-top:4% auto;
	}
	
}

@media screen and (max-width: 320px) {
	.select .remove {
		padding: 3px;
	}
}
!

JS 

              
                /**
 * @author Alicia Malacarne
 * 2017-2019
 * Script coded with JQuery framework, take care to call it in your HTML pages
 * this function displays a boxlist when the .select_input field is clicked
 * Call select() function in your main JQuery script
 */
$(document).ready(function () {
  select();
  //button click
  $('[type=\'button\']').click(function () {
    var form = $(this).parent();
    var datas = form.serialize();
    var txt = 'My favorite ' + datas;
    alert(txt);
  });
});
function select() {
  //display the list
  $('.select .select_input').click(function () {
    var width = $(this).width();
    var box = $(this).parent().find('.select_box');
    box.css('display', 'flex').css('width', width + 'px');
    resize_boxlist(box);
    $('#popup_content').append('<div id=\'fade_transparant\'></div>');
    $('#fade_transparant').css({
      'filter': 'alpha(opacity=80)'
    }).fadeIn();
  });
  //close list
  $('#popup_content').on('click', '#fade_transparant', function () {
    close_select();
  });
  //display sub list
  $('.select_box .select_group .select_header').click(function () {
    //hide main items list
    $('.select_group .select_item').css('display', 'none');
    if ($(this).attr('data-sslist') == 'show') {
      //remove color selection and close icon
      $(this).removeAttr('data-sslist').find('.remove').removeClass('remove').addClass('caret');
      //display last header
      $('.select_header').css('display', 'flex');
    } else {
      //remove data-sslist attribute from all headers for getting just one
      $('.select_header[data-sslist=\'show\']').removeAttr('data-sslist').find('.remove').removeClass('remove').addClass('caret');
      $(this).attr('data-sslist', 'show').find('.caret').removeClass('caret').addClass('remove');
      $(this).parent().find('.select_item').css('display', 'flex');
      //hide others headers
      $('.select_header:not([data-sslist="show"]').css('display', 'none');
    }
    var box = $(this).parents('.select_box');
    resize_boxlist(box);
  });
  //change dropinput when click on item_list
  $('.select_item').click(function () {
    var val = $(this).attr('data-valeur');
    //modify text display on select_input
    $(this).parents('.select').find('.select_input .select_text').val(val);
    //close list and fade
    close_select();
  });
} 

/**
 * close list and field
 */
function close_select() {
  //hide select_box on item click
  $('.select_box').css({
    'display': 'none',
    'top': '',
    'max_height': ''
  });
  //reset display list parameters
  $('.select_box').find('.select_header').css('display', 'flex');
  $('.select_group').find('.select_item').css('display', 'none');
  $('.select_header[data-sslist=\'show\']').removeAttr('data-sslist').find('.remove').removeClass('remove').addClass('caret');
  $('#fade_transparant').fadeOut();
  $('#fade_transparant').remove();
}

/**
 * resize boxlist according to its size and window size
 */
function resize_boxlist(box) {
  var boxSize = 0;
  var groups=box.find('.select_group');
  var nb = groups.length;
  if(nb>0){
    for (var i = 0; i < nb; i++) {
      var elem=$(groups[i]);
      boxSize = boxSize + elem.height();
    }
  }
  //check boxlist position
  var H = $(window).height();
  var h = (boxSize == 0) ? box.height()  : boxSize;
  var t = box.offset().top;
  var T = box.prev().offset().top;
  if (h < H) {
    if ((h + t) > H) {
      if (h < T) {
        t = T - h;
      } else {
        t = (H - h) / 2;
      }
    }else{
      if(t<T){
        t=T-h;
      }
    }
  } else {
    h = H;
    t = 0;
    box.css('overflow-y', 'scroll');
  }
  box.css({
    'top': t + 'px',
    'max-height': h + 'px'
  });
}
!
999px

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