js 关于dom 动态调用css

dom 动态调用css 发现 不可以通过document.getattributebyclassname 来调用style
<div class="box" id="box"> 你好呀</div>


 <script>
      
        var box = document.getElementsByClassName("box")[0];
   
        var box1 = document.getElementById("box");
        box.style.width = "200px";
    
        

    </script>
运行结果及报错内容
"resource": "/d:/html源码/js练习/20-dom概述/css动态设置.html",
"owner": "_generated_diagnostic_collection_name_#3",
"severity": 8,
"message": "An identifier or keyword cannot immediately follow a numeric literal.",
"source": "javascript",
"startLineNumber": 17,
"startColumn": 30,
"endLineNumber": 17,
"endColumn": 32
我想要达到的结果

想要弄明白为什么 通过id选择就可以达到