您可以通过去除边框、背景和填充来隐藏按钮;
<button type="submit" id="completed-task" class="fabutton">
<i class="fa fa-square-o fa-lg"></i>
</button>
.fabutton {
background: none;
padding: 0px;
border: none;
}
@import url('https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css');
.fabutton {
background: none;
padding: 0px;
border: none;
}
<button type="submit" id="completed-task" class="fabutton">
<i class="fa fa-square-o fa-lg"></i>
</button>