Why does unitless line-height behave differently from percentage or em in this example?

Question

I\'m perplexed by the behavior of the following CSS, also illustrated in this fiddle.

                  
        
        

          

            
            

              
              

            
    


                                 
              
            
                          
    

        
         

                验证码
                

                  
                
                

                   看不清?
                
              
                                  

                    
   
                 
             
              
提交回复