使用CSS无法获得平滑的下拉菜单

Question

Hi guys I've been having trouble getting my sub menu dropdown working with CSS. I'm trying to add a smooth transition appearance but at the moment the menu doesn't even display when I hover. I'm sure it's something small that I'm missing but I just can't seem to figure it out where the problem is. Here's the code:

#main-navigation ul.folder-child{
    opacity: 0;
    visibility: hidden;
    -moz-transition: height 0.3s ease-in-out;
    -ms-transition: height 0.3s ease-in-out;
    -o-transition: height 0.3s ease-in-out;
    -webkit-transition: height 0.3s ease-in-out;
    transition: height 0.3s ease-in-out;
}

#main-navigation li:hover ul.folder-child{
    opacity: 1;
    visibility: visible;
    top: 50px;
}

I'd appreciate any help anyone can offer. Thanks in advance!

Answer 1

You are defining transition for only height and there is no css rule defined for height. here is your solution.

Please Note: for transition of height property, you need to define height on normal and hover states.

ul.folder-child {
    width: 180px;
    height: 0;
    overflow: hidden;
    opacity: 0;
    visibility: hidden;

    position:absolute;
    top: 100%;
    left: 0;

    transition: all 0.4s ease-in-out;
    -o-transition: all 0.4s ease-in-out;
    -webkit-transition: all 0.4s ease-in-out;
}
#main-navigation li:hover > ul.folder-child {
    height: 100px;
    opacity: 1;
    visibility: visible;
}

Check out http://jsfiddle.net/logiccanvas/vWDvy/480/

Answer 2

也许检查您的浏览器，IE不支持所有CSS3功能。

Answer 3

Maybe it is better to use display: none and display: block instead of opacity and visibility ;

<ul>
        <li>
            <a href="#">Item #1</a>
            <ul>
                <li><a href="">Sub-Item #1</a></li>
                <li><a href="">Sub-Item #2</a></li>
                <li><a href="">Sub-Item #3</a></li>
            </ul>
        </li>
        <li>
            <a href="#">Item #2</a>
            <ul>
                <li><a href="">Sub-Item #4</a></li>
                <li><a href="">Sub-Item #5</a></li>
                <li><a href="">Sub-Item #6</a></li>
            </ul>
        </li>
        <li>
            <a href="#">Item #3</a>
            <ul>
                <li><a href="">Sub-Item #7</a></li>
                <li><a href="">Sub-Item #8</a></li>
                <li><a href="">Sub-Item #9</a></li>
            </ul>
        </li>
    </ul>

ul > li {display: block; float: left; margin-right: 10px; position: relative; background: Red; padding: 0.5em; line-height: 1em}
ul ul {display: none; width: 150px; position:absolute; top: 2em; left: 0}
ul ul > li {float: none;}
ul > li:hover > ul,
ul > a:hover + ul {display: block}

http://jsfiddle.net/spliter/vWDvy/

WORK ON IE ASWELL!