I'm using tidy 5.2.0.
The following works fine and shows no tidy linting errors:
<button type = "button" class = "btn btn-default">
<i class="glyphicon glyphicon-calendar"></i>
</button>
However, if I place the button in a span tag, tidy shows 3 errors:
<span class="input-group-btn">
<button type = "button" class = "btn btn-default">
<i class="glyphicon glyphicon-calendar"></i>
</button>
</span>
Error 1: missing </button>
Error 2: inserting implicit <span>
Error 3: replacing unexpected button with </button>