CSS :: before :: after类的伪元素不起作用 [英] CSS ::before ::after pseudo-element of class not working

问题描述

我正在尝试在菜单标题中添加 :: before 和 :: after 伪元素。对于菜单外的常规链接，伪元素可以正常工作。但是，当我尝试将它们应用于菜单项时，设置了 background 属性，但设置了 :: before 和 :: after 属性不是。

I am trying to add a ::before and ::after pseudo-element to a menu heading. The pseudo-elements work fine for a regular link outside of the menu. However, when I am trying to apply them to a menu item, the background property is set, but the ::before and ::after properties are not.

以下是相关的CSS：

#cssmenu {
    background-color: #FFFFCC;
    clear: both;
    display: inline;
    float: left;
    list-style: none;
    overflow: hidden;
    text-align: center;
    text-transform: uppercase;
    width: 100%;
}
#cssmenu li {
    display: inline-block;
}
#cssmenu li a {
    display: inline-block;
}
#cssmenu li ul {
    /*margin-top: 0px; submenu location relative to bottom of parent */
    display: none;
}
#cssmenu li:hover ul {
    display: block;
    position: absolute;
}
#cssmenu li ul li a {
    width: 200px;
}
#cssmenu li:hover > a {
    /* shadow around parent when hover */
    box-shadow: 5px 5px 25px #000;
    -moz-box-shadow: 5px 5px 25px #000;
    -webkit-box-shadow: 5px 5px 25px #000;
}
#cssmenu li a {
    color: #000;
    font-weight: bold;
    text-decoration: none;
    padding: 0 12px;
    line-height: 24px;
}
#cssmenu li a:hover {
    background-color: #99CC99;
    /*padding: 12px;  link background when hover over link */
}
#cssmenu li ul {
    background: rgba(255,255,255,0.9);  child menu background w/ transparency 
    padding: 10px 5px;

    box-shadow: 5px 5px 25px #BBB;
    -moz-box-shadow: 5px 5px 25px #BBB;
    -webkit-box-shadow: 5px 5px 25px #BBB;

    border-radius: 0px 15px 15px 15px;
    -moz-border-radius: 0px 15px 15px 15px;
    -webkit-border-radius: 0px 5px 5px 5px;
}
/* display sub-menu as list */
#cssmenu li ul li {
    display: block;
    text-align: left;
}
#cssmenu li ul li a, #nav li ul li a:hover {
    background: transparent;
    color: #000;
    width: 180px;
    font-size: 0.95em;
    font-weight: normal;
    padding: 6px;
}
#cssmenu li ul li a:hover {
    /*text-decoration: underline;*/
    box-shadow: none;
    -moz-box-shadow: none;
    -webkit-box-shadow: none;
    border-radius: 0;
    -moz-border-radius: 0;
    -webkit-border-radius: 0;
}
.menuheader {
    /* allow for the pseudo-elements which do not have layout due to absolute positioning */
    margin: 12px 15px;
    border: 0;
    background: url("../../../images/buttonslice24.png") 24px 0 repeat-x ;
}
.menuheader::before,
.menuheader::after {
    content: ' ';
    position: absolute;
    top: 0;
    width: 12px;
    height: 24px;
}
.menuheader::before {
    left: -12px;
    background: url("../../../images/buttonleft24.png") 0 0;
    no-repeat;
}
.menuheader::after {
    right: -12px;
    background: url("../../../images/buttonright24.png") 100% 0;
    no-repeat;
}

以及相关的HTML：

<div id="cssmenu">
  <ul>
    <li><a class="menuheader" href="../../../contact-us.aspx">Contact</a></li>
    <li><a class="menuheader" href="../../../">Home</a></li>
    <li><a class="menuheader">Store</a>
      <ul>
        <li><a href="../../../shipping-policy.aspx">Shipping &amp; Return Policy</a></li>
      </ul>
    </li>
    <li><a class="menuheader" href="../../../account.aspx">Account</a></li>
    <li><a class="menuheader" href="../../../cart.aspx">View Cart</a></li>
  </ul>
</div>

JSFiddle

我还尝试通过 #cssmenu引用 menuheader 类项目ul li a 代替，并且实际上可行（有点）。而不是菜单项本身旁边显示的 :: before 和 :: after 图像，它们显示在菜单项旁边。

I have also tried to reference the menuheader class items by #cssmenu ul li a instead, and that actually works (sort of). Instead of the ::before and ::after images showing next to the menu item itself, they show next to the first item in the drop-down menu.

我在这里还阅读了其他有关伪元素以及http://www.w3schools.com/css/css_pseudo_elements.asp ，据我所知， :: before 和 :: after 应该能够应用于< a> 元素。这可能是一个简单的问题，但是我在这里没有看到这个问题。

I have read other questions on here regarding pseudo-elements, as well as http://www.w3schools.com/css/css_pseudo_elements.asp and as far as I can tell, the ::before and ::after should be able to be applied to an <a> element. This may be some simple issue, but I am just not seeing the issue here.

推荐答案

如果您要绝对定位伪元素，则需要为其父元素赋予位置值，与非伪元素相同。

If you're going to absolutely position the pseudo elements, you need to give their parent a position value, same as with non-pseudo elements.

只需添加：

.menuheader {
    position : relative;
}

这是JSFiddle的更新版本（唯一的变化是在上面添加了代码，并使伪元素的背景变为纯色，以供演示）： http://jsfiddle.net/99Aq8/1/

Here is an updated version of your JSFiddle (the only changes are adding the above code and making the background of the pseudo elements a solid color for demonstration): http://jsfiddle.net/99Aq8/1/

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