question mark and exclamation mark blinking in checkbox html instead of tick box

i have created a checkbox list group from bootstrap, the code is like below:

<link href="https://www.ishtasakhi.com/assets/css/bootstrap-checkbox-radio-list-group-item.css" rel="stylesheet" />
<link href="https://www.ishtasakhi.com/assets/css/bootstrap-checkbox-radio-list-group-item.min.css" rel="stylesheet" />
<div class="list-group checkbox-list-group">
  <div style="height:45px;" class="list-group-item">&nbsp;<label><input type="checkbox"><span class="list-group-item-text"><i class="fa fa-fw"></i> Large</span></label></div>
  <div style="height:45px;" class="list-group-item">&nbsp;<label><input type="checkbox"><span class="list-group-item-text"><i class="fa fa-fw"></i> Medium</span></label></div>
</div>

now instead of tickbox, the checkbox is showing question mark and exclamatory mark blinking like below,

this is how it actually should look like demo can anyone please tell me what could be wrong here. thanks in advance