<!DOCTYPE html>
<head>
      <meta charset="UTF-8">
      <meta name="viewport" content="width=device-width, initial-scale=1.0">
      <title>Document</title>
      <style>
            .btnstyle{
                background: yellow;
                font-size: 30px;
            }
      </style>
      <script>
            window.onload = function(){
                  var oBtn =  document.getElementById("btn1");
                  alert(oBtn.type);
            //       alert(oBtn.value);
                  // 设置标签属性
                  oBtn.name = "username";
                  // 设置样式属性
            //       oBtn.style.background = "red";
                  // 相当于设置class，以后通过类选择器给标签添加样式
                  // 注意点：class这里变成className
                  oBtn.className = "btnstyle";
            //       oBtn.style.fontSize = "30px";
            };
      </script>
</head>
<body>
     <input type="button" value="按钮" id="btn1">
</body>
</html>

 

 

 

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        window.onload = function(){
            // 根据标签的id获取标签对象
            var oDiv = document.getElementById("div1")
            // 获取标签内容
            alert(oDiv.innerHTML)
            // 设置标签内容
            oDiv.innerHTML = "<a href='http://www.baidu.com'>百度</a>"
        }
    </script>
</head>
<body>
    <div id="div1">
        哈哈哈哈
    </div>
</body>
</html>