The fourth day of learning Div CSS in ten days [vertical navigation and secondary menu]_html/css

1. Vertical list
Vertical list, or vertical navigation, is widely used in product lists on websites, such as the Taobao service on the left side of Taobao. Today we will learn about vertical navigation Production

First create a new page, then insert a div with the ID menu, then select the text in the design view, click the ul icon on the toolbar, ul and li will be automatically inserted, and then modify the text content as The content you need.

<div id="menu"><ul><li>首页</li><li>网页版式布局</li><li>div+css教程</li><li>div+css实例</li><li>常用代码</li><li>站长杂谈</li><li>技术文档</li><li>资源下载</li><li>图片素材</li></ul></div>

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From the preview, there are large gaps all around, and there is a dot in front of each line , this is caused by the default style of the label. Next we need to create a style sheet to clear the default style of the label
The CSS code is as follows:

<style type="text/css">#menu ul { list-style: none; margin: 0px; padding: 0px; }</style>

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Complete HTML The code is as follows:

<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=gb2312" /><style type="text/css">body { font-family: Verdana; font-size: 12px; line-height: 1.5; }a { color: #000; text-decoration: none; }a:hover { color: #F00; }#menu { width: 100px; border: 1px solid #CCC; }#menu ul { list-style: none; margin: 0px; padding: 0px; }#menu ul li { background: #eee; padding: 0px 8px; height: 26px; line-height: 26px; border-bottom: 1px solid #CCC; }</style></head></p><p><body><div id="menu"><ul><li><a href="#">首页</a></li><li><a href="#">网页版式布局</a></li><li><a href="#">div+css教程</a></li><li><a href="#">div+css实例</a></li><li><a href="#">常用代码</a></li><li><a href="#">站长杂谈</a></li><li><a href="#">技术文档</a></li><li><a href="#">资源下载</a></li><li><a href="#">图片素材</a></li></ul></div></body></html>

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2. The default style of labels
Most labels have their own default styles, such as what I encountered in the course the next day. The body has default outer margins. In this example, the dot before ul and the inner margin on the left side. In addition, h1-h6 font sizes are different. em defaults to italic, and strong means bold. Because of these default styles, a well-designed page can be easily read by users even if no styles are loaded. But these default styles are of no use to us at this time, so they need to be cleared. For convenience, it is recommended to use tag redefinition, which can easily unify the global styles. In addition, the pictures on the page will have a border added by default after adding links, and ul will add dots in front of the list by default, which need to be removed.

body, ul, li, h1, h2, h3, h4, h5, h6, p, form, dl, dt, dd { margin: 0px; padding: 0px; font-size: 12px; font-weight: normal; }ul { list-style: none; }img { border-style: none; }

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3. CSS derived selector
CSS beginners don’t know that using sub-selectors is one of the reasons that affects their efficiency. Derived selectors can help you save a lot of class definitions. In the example above, I used some derived selectors with the following css code

#menu ul { list-style: none; margin: 0px; padding: 0px; }#menu ul li { background: #eee; padding: 0px 8px; height: 26px; line-height: 26px; border-bottom: 1px solid #CCC; }

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#menu ul and #menu ul li are derived selectors. If we remove the #menu in front, it will redefine the ul tag, and the redefined attributes will be applied globally. After adding #menu in front, it will define the style of ul in the menu element with ID, and set its The style only takes effect on the ul under #menu, not the ul after it. This is a bit like a local variable in programming, and directly defining ul is equivalent to a global variable. #menu ul li is defined as the li under ul within the menu element. The derived selector allows us to no longer need to define a style name for each li to define the style. We only need to use the derived selector to select from its parent element. That’s it, this can greatly improve efficiency.

4. Grouping of css selectors
You can group selectors so that grouped selectors can share the same statement. Use commas to separate selectors that need to be grouped. In the example below, we have grouped all heading elements. All title elements are green, and p paragraphs, div partitions, and spans are all 20 pixel fonts.

h1,h2,h3,h4,h5,h6 { color: green; } p,div,span{ font-size:20px; }

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5. Vertical secondary list
The secondary menu means that when the mouse is placed on the first-level menu, the corresponding secondary menu will pop up. Level menu will disappear automatically after removing the mouse. Let’s modify the above example. The code is as follows:

<div id="menu"><ul><li><a href="@#">首页</a></li><li><a href="#">网页版式布局</a><ul><li><a href="#">自适应宽度</a></li><li><a href="#">固定宽度</a></li></ul></li><li><a href="#">div+css教程</a><ul><li><a href="#">新手入门</a></li><li><a href="#">视频教程</a></li><li><a href="#">常见问题</a></li></ul></li><li><a href="#">div+css实例</a></li><li><a href="#">常用代码</a></li><li><a href="#">站长杂谈</a></li><li><a href="#">技术文档</a></li><li><a href="#">资源下载</a></li><li><a href="#">图片素材</a></li></ul></div>

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All HTML JS CSS:

<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=gb2312" /><script type="text/javascript"><!--//--><![CDATA[//><!--startList = function() {if (document.all&&document.getElementById) {navRoot = document.getElementById("menu");var allli = navRoot.getElementsByTagName("li")for (i=0; i<allli.length; i++) {node = allli[i];node.onmouseover=function() {this.className+=" current";}node.onmouseout=function() {this.className=this.className.replace(" current", "");}}}}window.onload=startList;//--><!]]></script><style type="text/css">body { font-family: Verdana; font-size: 12px; line-height: 1.5; }img { border-style: none; }a { color: #000; text-decoration: none; }a:hover { color: #F00; }#menu { width: 100px; border: 1px solid #CCC; border-bottom:none;}#menu ul { list-style: none; margin: 0px; padding: 0px; }#menu ul li { background: #eee; padding: 0px 8px; height: 26px; line-height: 26px; border-bottom: 1px solid #CCC; position:relative; }#menu ul li ul { display:none; position: absolute; left: 100px; top: 0px; width:100px; border:1px solid #ccc; border-bottom:none; }#menu ul li.current ul { display:block;}#menu ul li:hover ul { display:block;}</style></head></p><p><body><div id="menu"><ul><li><a href="#">首页</a></li><li><a href="#">网页版式布局</a><ul><li><a href="#">自适应宽度</a></li><li><a href="#">固定宽度</a></li></ul></li><li><a href="#">div+css教程</a><ul><li><a href="#">新手入门</a></li><li><a href="#">视频教程</a></li><li><a href="#">常见问题</a></li></ul></li><li><a href="#">div+css实例</a></li><li><a href="#">常用代码</a></li><li><a href="#">站长杂谈</a></li><li><a href="#">技术文档</a></li><li><a href="#">资源下载</a></li><li><a href="#">图片素材</a></li></ul></div></body></html>

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6. Relative positioning and absolute positioning
Positioning tag: position
Contains attributes: relative (relative) absolute (absolute)
1.position:relative; If an element is positioned relatively, first it will appear at its location. Then move the element "relative to" its original starting point by setting a vertical or horizontal position. (One more point, when positioned relatively, the element still occupies the original space regardless of whether it is moved. Therefore, moving the element will cause it to cover other boxes)

2.position:absolute; means absolute positioning, and the position will be based on Starting from the upper left corner of the browser. Absolute positioning takes the element out of the document flow so it doesn't take up space. Elements in normal document flow are laid out as if absolutely positioned elements were not present. (Because absolutely positioned boxes have nothing to do with document flow, they can cover other elements on the page and their hierarchical order can be controlled by z-index. The higher the z-index, the further up it appears.)

3. After the parent container uses relative positioning and the child element uses absolute positioning, the position of the child element is no longer relative to the upper left corner of the browser, but relative to the upper left corner of the parent window

4. Relative Positioning and absolute positioning need to be used with top, right, bottom, and left to locate the specific position. These four attributes only take effect after the element is positioned, and are invalid in other cases. In addition, these four attributes can only use two adjacent ones at the same time. You cannot use up and down at the same time, or use left and right at the same time.