What is the difference between width property 100% and auto in css?

 width:auto: The default width value of block-level elements. When set to this value, the browser will automatically select an appropriate width value to adapt to the width of the parent element. When the width is set to 100%, the width of the child element box The value is equal to the parent's content, and as the parent's content automatically changes, after adding the padding and margin of the child element, its width remains unchanged, which is the difference from setting it to auto. But we most often use width:auto, because it is more flexible, width:100% is used less, because when adding padding or margin, this method is easy to make it exceed the parent box and destroy the original layout.

The following is the actual code demonstration, the picture intuitively reflects the difference between the two.

<style>

        .outer{

            background-color: aquamarine;

            width: 200px;

            height: 200px;

        }

        .inner1{

            height: 50px;

            width: auto;

            background-color: pink;

            padding: 10px;

            border: 5px solid red;

            margin: 5px;

        }

        .inner2{

            height: 50px;

            width: 100%;

            padding: 10px;

            background-color: yellow;

            border: 5px solid red;

            margin: 5px;

        }

    </style>

</head>

<body>

    <div class="outer">

        <div class="inner1">

        </div>

        <hr>

        <div class="inner2">

           

        </div>

    </div>