I am encountering an issue with the height of a button element generated by Bootstrap Select within an HTML select element. When using HTML5 and including the declaration as the first line, the button renders correctly with a height of 24.
However, when HTML5 is not used, the button's height is only 8.875 on Blink (Opera 56.0) and 8.8833 on Gecko (Firefox 60.3), resulting in a distorted display.
I have tested this behavior without any server configurations, simply by saving the code snippet below as an HTML file and toggling the presence of the declaration:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.2/css/bootstrap-select.min.css">
<title>Hello, world!</title>
</head>
<body>
<h1>Hello, world!</h1>
<div class="container">
<div class="form-group row">
<label for="foobar" class="col-sm-2 col-form-label">Sauce</label>
<div class="col-sm-10">
<select class="form-control selectpicker">
<option>Mustard</option>
<option>Ketchup</option>
<option>Barbecue</option>
</select>
</div>
</div>
</div>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.2/js/bootstrap-select.min.js"></script>
</body>
</html>