3-3 組件

1. Glyphicons 字體圖標：http://v3.bootcss.com/components/#glyphicons

   <span  class= "glyphicon glyphicon-search"  aria-hidden= "true" ></span>

   應用在按鈕

   <button  type= "button"  class= "btn btn-default"  aria-label= "Left Align" > 
     <span  class= "glyphicon glyphicon-align-left"  aria-hidden= "true" ></span> 
   </ button>
   
   <button  type= "button"  class= "btn btn-default btn-lg" > 
     <span  class= "glyphicon glyphicon-star"  aria-hidden= "true" ></span> Star
    </button>

2. alert組件中所包含的圖標是用來表示這是一條錯誤消息的，通過添加額外的.sr-only文本就可以讓輔助設備知道這條提示所要表達的意思了。

   <div  class= "alert alert-danger"  role= "alert" > 
     <span  class= "glyphicon glyphicon-exclamation-sign"  aria-hidden= "true" ></span> 
     <span  class= "sr-only" > Error: </span>
     Enter a valid email address
   </div>