对于nav元素，应将padding设置为0px：

nav {
  background-color: #8080ff;
  margin: 0px;
  position: absolute;
  padding: 0px !important;
}

• • 这就是"!important是什么意思"的答案**

CSS中的!important规则用于增加属性/值的重要性。
事实上，如果您使用!important规则，它将覆盖该元素上该特定属性的所有以前的样式规则!

• • 示例：**

#id {
  background-color: blue;
}

.class {
  background-color: gray;
}

p {
  background-color: red !important;
}

或者它将优先CSS属性。!重要的有最高的优先级比什么都重要。

添加填充：0 px！对CSS中的导航选择器很重要

遵循以下代码

<style>
    li.nav-item{
        display: contents;
        padding: 12px;
        margin-right: 12px;
        top: 0;
        position: absolute;
    }
    li.nav-item a{
        margin-right: 12px;
        
    }
    li.nav-item a:hover{
        background-color: #cccccc;
        /* display: contents; */
        padding: 16px;
        top: 0;
        /* position: absolute; */
    }
  
    html {
      position: relative;
      min-height: 100%;
      margin: 0;
    }
    
    body {
      background-color: #cccccc;
      margin: 0;
      /*margin-bottom: 60px;*/
    }
    
    nav {
        background-color: #8080ff;
        margin: 0px;
        position: absolute;
        width: 100%;
    }
    
    .navbar-nav {
        height: 100%;
        margin: 0;
    }
    ul{
        margin-left: 60px !important;
    }
</style>

<nav class="navbar navbar-expand-sm navbar-toggleable-sm navbar-light border-bottom box-shadow mb-3">
    <div class="container" style="overflow:auto">
        <a class="navbar-brand" asp-area="" asp-page="/Index">BookStore</a>
        <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target=".navbar-collapse" aria-controls="navbarSupportedContent"
                aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>
        <div class="navbar-collapse collapse d-sm-inline-flex justify-content-between">
            <ul class="navbar-nav flex-grow-1">
                <li class="nav-item">
                    <a class="nav-link text-dark" asp-area="" asp-page="/Index">Home</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link text-dark" asp-area="" asp-page="/Books/Index">Books</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link text-dark" asp-area="" asp-page="/Privacy">Privacy</a>
                </li>
            </ul>
        </div>
    </div>
</nav>