做為相對長度單位的倆個活寶， 都是以倍數關係定長度，只是參照物不一樣，這裡就案例分析兩者的區別：
1，em 的參照倍數對象為其自身字型大小，font-size;如果沒有向上繼承；
2，rem的參照倍數對象為根項目
案例代碼：
<html style="font-size:14px"><div style="font-size:14px;background:#bbb;border:1px solid #f00;margin-bottom:5px;"> this is a 14px div <p style="font-size:1em;background:#ccc;"> this is a 1em p in 14px div </p> </div> <div style="font-size:10px;background:#b71119;border:1px solid #006f00;color:#fff;margin-bottom:5px"> this is a 10px div <p style="font-size:2em;background:#b0b000"> this is a 2em p in 10px div <span style="font-size:1em;color:#b71119"> span: this is a 1em span in 2emp of 10px div; </span> </p> </div> <div style="font-size:1rem;background:#ee7324;padding-bottom: 1px;"> this is 1rem div <p style="font-size:2rem;background: #eee"> this is a 2rem p in 1rem div <span style="font-size:.5rem;color:green">span: this is a 0.5rem span in 2rem p of 1rem div</span> </p> </div>
使用：
如果是跟父元素成倍數關係縮放大小，可選擇em為單位；
rem常用在移動端開發，是非常不錯的選擇，可解決不同螢幕下的顯示不適應情況，後面會介紹如何使用100%布局和rem 來開發移動端業務。
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