<div contenteditable>我是測試文字</div>    

document.execCommand(String aCommandName, Boolean aShowDefaultUI, String aValueArgument)
//aCommandName為命令名稱，不可省略
//aShowDefaultUI為是否展示用戶界面，默認為false，可省略
//aValueArgument為額外參數(shù)值，默認為null,可省略

document.execCommand(''justifyCenter'');

document.execCommand(''justifyLeft'');

document.execCommand(''justifyRight'');

document.execCommand(''indent'');

document.execCommand(''outdent'');

document.execCommand(''fontname'',false,sFontName)

document.execCommand(''fontsize'',false,sFontSize)

document.execCommand(''forecolor'',false,sFontColor)

document.execCommand(''backColor'',false,sBackColor)

document.execCommand(''bold'');

document.execCommand(''italic'');

document.execCommand(''underline'');

document.execCommand(''copy'');

document.execCommand(''cut'');

document.execCommand(''paste'');

document.execCommand(''selectAll'');    

document.execCommand(''delete'');

document.execCommand(''forwarddelete'');

document.execCommand(''removeFormat'');

document.execCommand(''redo'');

document.execCommand(''undo'');

document.execCommand(''print'');

document.execCommand(''insertImage'',false,''image.png'');

<div class="box">
    <div class="con" id="con">
        <button data-name="selectAll">全選</button>
        <button data-name="delete">刪除</button>
        <button data-name="undo">撤銷</button>
        <button data-name="print">打印</button>
        <button data-name="bold">加粗</button>
        <button data-name="italic">斜線</button>
        <button data-name="underline">下劃線</button>        
        <button data-name="fontsize" data-value="16px">大號字體</button>
        <button data-name="forecolor" data-value="red">紅色文本</button>
        <button data-name="backcolor" data-value="gray">灰色背景</button>        
        <button data-name="removeFormat">清空格式</button>
    </div>
    <div class="show" id="show" contenteditable>我是測試文字</div>
</div>    

.box{
    width: 500px;
}
.con{
    overflow:hidden;
    margin-bottom: 6px;
}
.con button{
    float: left;
    padding: 2px;
    border: 1px solid gray;
    margin-right: 2px;
    cursor: pointer;
}
.show{
    height: 200px;
    border: 2px solid rgba(0,0,0,0.3);
}

<script>
var aCon = document.getElementById(''con'').getElementsByTagName(''button'');
for(var i = 0; i < aCon.length; i++){
    aCon[i].onclick = function(){
        document.execCommand(this.dataset.name,false,this.dataset.value);
    }    
}
</script>