老师，给body标签设置width属性后，为什么字体显示宽度收到限制，而背景颜色宽度不受限制呢？

<style type="text/css">
        body{
           
            width: 900px;
            background-color: yellow;
            background-image: url(img/5ea016c108c1f40603610240.jpg);
            background-position: center center;
            background-repeat: no-repeat;
        }
        p{
            background-color: red;
        }
        p{
         font-size: 40px;
      }
      .p1{
         font-family: 隶书;
      }
    </style>