一、代码
<!DOCTYPE html>
<html lang="zh-cn">
<head>
<title>css默认值 编程笔记 html5&css&js</title>
<meta charset="utf-8" />
<style>
body {
color: cyan;
background-color: teal;
}
table {
margin: auto;
width: 80%;
font-family: "Microsoft Yahei";
word-wrap: break-word;
text-align: justify;
border-collapse: collapse;
font-size: 18px;
border-width: 1px;
border-style: solid;
border-color: black;
width: 60%;
}
table th {
height: 50px;
font-size: 28px;
}
table td {
height: 24px;
font-size: 18px;
}
table th,
table td {
padding: 8px 8px;
border-color: orange;
border-style: solid;
border-collapse: collapse;
box-sizing: 1px;
border-width: 1px;
}
</style>
</head>
<body>
<table>
<tbody>
<tr>
<th width="30%">元素</th>
<th width="70%">默认的 CSS 值</th>
</tr>
<tr>
<td>a:link</td>
<td>
<ul>
<li>color: (internal value);</li>
<li>text-decoration: underline;</li>
<li>cursor: auto;</li>
</ul>
</td>
</tr>
<tr>
<td>a:visited</td>
<td>
<ul>
<li>color: (internal value);</li>
<li>text-decoration: underline;</li>
<li>cursor: auto;</li>
</ul>
</td>
</tr>
<tr>
<td>a:link:active</td>
<td>color: (internal value);</td>
</tr>
<tr>
<td>a:visited:active</td>
<td>color: (internal value);</td>
</tr>
<tr>
<td>abbr</td>
<td>None.</td>
</tr>
<tr>
<td>address</td>
<td>
<ul>
<li>display: block;</li>
<li>font-style: italic;</li>
</ul>
</td>
</tr>
<tr>
<td>area</td>
<td>display: none;</td>
</tr>
<tr>
<td>article</td>
<td>display: block;</td>
</tr>
<tr>
<td>aside</td>
<td>display: block;</td>
</tr>
<tr>
<td>audio</td>
<td>None.</td>
</tr>
<tr>
<td>b</td>
<td>font-weight: bold;</td>
</tr>
<tr>
<td>base</td>
<td>None.</td>
</tr>
<tr>
<td>bdi</td>
<td>None.</td>
</tr>
<tr>
<td>bdo</td>
<td>unicode-bidi: bidi-override;</td>
</tr>
<tr>
<td>blockquote</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 40px;</li>
<li>margin-right: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>body</td>
<td>
<ul>
<li>display: block;</li>
<li>margin: 8px;</li>
</ul>
</td>
</tr>
<tr>
<td>body:focus</td>
<td>outline: none;</td>
</tr>
<tr>
<td>br</td>
<td>None.</td>
</tr>
<tr>
<td>button</td>
<td>None</td>
</tr>
<tr>
<td>canvas</td>
<td>None.</td>
</tr>
<tr>
<td>caption</td>
<td>
<ul>
<li>display: table-caption</li>
<li>text-align: center;</li>
</ul>
</td>
</tr>
<tr>
<td>cite</td>
<td>font-style: italic;</td>
</tr>
<tr>
<td>code</td>
<td>font-family: monospace;</td>
</tr>
<tr>
<td>col</td>
<td>display: table-column;</td>
</tr>
<tr>
<td>colgroup</td>
<td>display: table-column-group</td>
</tr>
<tr>
<td>datalist</td>
<td>display: none;</td>
</tr>
<tr>
<td>dd</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-left: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>del</td>
<td>text-decoration: line-through;</td>
</tr>
<tr>
<td>details</td>
<td>display: block;</td>
</tr>
<tr>
<td>dfn</td>
<td>font-style: italic;</td>
</tr>
<tr>
<td>dialog</td>
<td>None.</td>
</tr>
<tr>
<td>div</td>
<td>display: block;</td>
</tr>
<tr>
<td>dl</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
</ul>
</td>
</tr>
<tr>
<td>dt</td>
<td>display: block;</td>
</tr>
<tr>
<td>em</td>
<td>font-style: italic;</td>
</tr>
<tr>
<td>embed:focus</td>
<td>outline: none;</td>
</tr>
<tr>
<td>fieldset</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-left: 2px;</li>
<li>margin-right: 2px;</li>
<li>padding-top: 0.35em;</li>
<li>padding-bottom: 0.625em;</li>
<li>padding-left: 0.75em;</li>
<li>padding-right: 0.75em;</li>
<li>border: 2px groove (internal value);</li>
</ul>
</td>
</tr>
<tr>
<td>figcaption</td>
<td>display: block;</td>
</tr>
<tr>
<td>figure</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 40px;</li>
<li>margin-right: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>footer</td>
<td>display: block;</td>
</tr>
<tr>
<td>form</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 0em;</li>
</ul>
</td>
</tr>
<tr>
<td>h1</td>
<td>
<ul>
<li>display: block;</li>
<li>font-size: 2em;</li>
<li>margin-top: 0.67em;</li>
<li>margin-bottom: 0.67em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>h2</td>
<td>
<ul>
<li>display: block;</li>
<li>font-size: 1.5em;</li>
<li>margin-top: 0.83em;</li>
<li>margin-bottom: 0.83em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>h3</td>
<td>
<ul>
<li>display: block;</li>
<li>font-size: 1.17em;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>h4</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 1.33em;</li>
<li>margin-bottom: 1.33em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>h5</td>
<td>
<ul>
<li>display: block;</li>
<li>font-size: .83em;</li>
<li>margin-top: 1.67em;</li>
<li>margin-bottom: 1.67em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>h6</td>
<td>
<ul>
<li>display: block;</li>
<li>font-size: .67em;</li>
<li>margin-top: 2.33em;</li>
<li>margin-bottom: 2.33em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>font-weight: bold;</li>
</ul>
</td>
</tr>
<tr>
<td>head</td>
<td>display: none;</td>
</tr>
<tr>
<td>header</td>
<td>display: block;</td>
</tr>
<tr>
<td>hr</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 0.5em;</li>
<li>margin-bottom: 0.5em;</li>
<li>margin-left: auto;</li>
<li>margin-right: auto;</li>
<li>border-style: inset;</li>
<li>border-width: 1px;</li>
</ul>
</td>
</tr>
<tr>
<td>html</td>
<td>
<ul>
<li>display: block;</li>
<li>html:focus</li>
<li>outline: none;</li>
</ul>
</td>
</tr>
<tr>
<td>i</td>
<td>font-style: italic;</td>
</tr>
<tr>
<td>iframe:focus</td>
<td>outline: none;</td>
</tr>
<tr>
<td>iframe[seamless]</td>
<td>display: block;</td>
</tr>
<tr>
<td>img</td>
<td>display: inline-block;</td>
</tr>
<tr>
<td>input</td>
<td>None.</td>
</tr>
<tr>
<td>ins</td>
<td>text-decoration: underline;</td>
</tr>
<tr>
<td>kbd</td>
<td>font-family: monospace;</td>
</tr>
<tr>
<td>label</td>
<td>cursor: default;</td>
</tr>
<tr>
<td>legend</td>
<td>
<ul>
<li>display: block;</li>
<li>padding-left: 2px;</li>
<li>padding-right: 2px;</li>
<li>border: none;</li>
</ul>
</td>
</tr>
<tr>
<td>li</td>
<td>display: list-item;</td>
</tr>
<tr>
<td>link</td>
<td>display: none;</td>
</tr>
<tr>
<td>main</td>
<td>None.</td>
</tr>
<tr>
<td>map</td>
<td>display: inline;</td>
</tr>
<tr>
<td>mark</td>
<td>
<ul>
<li>background-color: yellow;</li>
<li>color: black;</li>
</ul>
</td>
</tr>
<tr>
<td>menu</td>
<td>
<ul>
<li>display: block;</li>
<li>list-style-type: disc;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>padding-left: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>menuitem</td>
<td>None.</td>
</tr>
<tr>
<td>meta</td>
<td>None.</td>
</tr>
<tr>
<td>meter</td>
<td>None.</td>
</tr>
<tr>
<td>nav</td>
<td>display: block;</td>
</tr>
<tr>
<td>noscript</td>
<td>None.</td>
</tr>
<tr>
<td>object:focus</td>
<td>outline: none;</td>
</tr>
<tr>
<td>ol</td>
<td>
<ul>
<li>display: block;</li>
<li>list-style-type: decimal;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>padding-left: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>optgroup</td>
<td>None.</td>
</tr>
<tr>
<td>option</td>
<td>None.</td>
</tr>
<tr>
<td>output</td>
<td>display: inline;</td>
</tr>
<tr>
<td>p</td>
<td>
<ul>
<li>display: block;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
</ul>
</td>
</tr>
<tr>
<td>param</td>
<td>display: none;</td>
</tr>
<tr>
<td>picture</td>
<td>None.</td>
</tr>
<tr>
<td>pre</td>
<td>
<ul>
<li>display: block;</li>
<li>font-family: monospace;</li>
<li>white-space: pre;</li>
<li>margin: 1em 0;</li>
</ul>
</td>
</tr>
<tr>
<td>progress</td>
<td>None.</td>
</tr>
<tr>
<td>q</td>
<td>display: inline;</td>
</tr>
<tr>
<td>q::before</td>
<td>content: open-quote;</td>
</tr>
<tr>
<td>q::after</td>
<td>content: close-quote;</td>
</tr>
<tr>
<td>rp</td>
<td>None.</td>
</tr>
<tr>
<td>rt</td>
<td>line-height: normal;</td>
</tr>
<tr>
<td>ruby</td>
<td>None.</td>
</tr>
<tr>
<td>s</td>
<td>text-decoration: line-through;</td>
</tr>
<tr>
<td>samp</td>
<td>font-family: monospace;</td>
</tr>
<tr>
<td>script</td>
<td>display: none;</td>
</tr>
<tr>
<td>section</td>
<td>display: block;</td>
</tr>
<tr>
<td>select</td>
<td>None.</td>
</tr>
<tr>
<td>small</td>
<td>font-size: smaller;</td>
</tr>
<tr>
<td>source</td>
<td>None.</td>
</tr>
<tr>
<td>span</td>
<td>None.</td>
</tr>
<tr>
<td>strike</td>
<td>text-decoration: line-through;</td>
</tr>
<tr>
<td>strong</td>
<td>font-weight: bold;</td>
</tr>
<tr>
<td>style</td>
<td>display: none;</td>
</tr>
<tr>
<td>sub</td>
<td>
<ul>
<li>vertical-align: sub;</li>
<li>font-size: smaller;</li>
</ul>
</td>
</tr>
<tr>
<td>summary</td>
<td>display: block;</td>
</tr>
<tr>
<td>sup</td>
<td>
<ul>
<li>vertical-align: super;</li>
<li>font-size: smaller;</li>
</ul>
</td>
</tr>
<tr>
<td>table</td>
<td>
<ul>
<li>display: table;</li>
<li>border-collapse: separate;</li>
<li>border-spacing: 2px;</li>
<li>border-color: gray;</li>
</ul>
</td>
</tr>
<tr>
<td>tbody</td>
<td>
<ul>
<li>display: table-row-group;</li>
<li>vertical-align: middle;</li>
<li>border-color: inherit;</li>
</ul>
</td>
</tr>
<tr>
<td>td</td>
<td>
<ul>
<li>display: table-cell;</li>
<li>vertical-align: inherit;</li>
</ul>
</td>
</tr>
<tr>
<td>template</td>
<td>None.</td>
</tr>
<tr>
<td>textarea</td>
<td>None.</td>
</tr>
<tr>
<td>tfoot</td>
<td>
<ul>
<li>display: table-footer-group;</li>
<li>vertical-align: middle;</li>
<li>border-color: inherit;</li>
</ul>
</td>
</tr>
<tr>
<td>th</td>
<td>
<ul>
<li>display: table-cell;</li>
<li>vertical-align: inherit;</li>
<li>font-weight: bold;</li>
<li>text-align: center;</li>
</ul>
</td>
</tr>
<tr>
<td>thead</td>
<td>
<ul>
<li>display: table-header-group;</li>
<li>vertical-align: middle;</li>
<li>border-color: inherit;</li>
</ul>
</td>
</tr>
<tr>
<td>time</td>
<td>None.</td>
</tr>
<tr>
<td>title</td>
<td>display: none;</td>
</tr>
<tr>
<td>tr</td>
<td>
<ul>
<li>display: table-row;</li>
<li>vertical-align: inherit;</li>
<li>border-color: inherit;</li>
</ul>
</td>
</tr>
<tr>
<td>track</td>
<td>None.</td>
</tr>
<tr>
<td>u</td>
<td>text-decoration: underline;</td>
</tr>
<tr>
<td>ul</td>
<td>
<ul>
<li>display: block;</li>
<li>list-style-type: disc;</li>
<li>margin-top: 1em;</li>
<li>margin-bottom: 1 em;</li>
<li>margin-left: 0;</li>
<li>margin-right: 0;</li>
<li>padding-left: 40px;</li>
</ul>
</td>
</tr>
<tr>
<td>var</td>
<td>font-style: italic;</td>
</tr>
<tr>
<td>video</td>
<td>None.</td>
</tr>
<tr>
<td>wbr</td>
<td>None.</td>
</tr>
</tbody>
</table>
</body>
</html>
二、解释
CSS默认值（也称为浏览器默认样式）是指当HTML元素没有应用任何外部CSS样式时，浏览器自动为这些元素赋予的一组基本样式。这些样式是由浏览器的默认样式表（User Agent stylesheet）定义的，不同的浏览器可能会有细微的差异，但大多数浏览器在样式上保持了较高的一致性。
CSS默认值的目的是确保网页即使在没有CSS的情况下也能呈现出一定的可读性和外观。这些默认值包括了字体大小、颜色、边距、填充、列表样式等多种属性。例如：
- 所有的HTML元素都会有一个默认的字体大小（通常是16像素）。
- 链接（
<a>标签）默认会有不同的颜色（通常是蓝色），并且带有下划线，以区分它们是可以点击的。 - 段落（
<p>标签）默认会有上下一定的边距。 - 列表（
<ul>,<ol>标签）会有默认的列表项标记和缩进。
了解和使用CSS默认值对于网页开发者来说很重要，因为它可以帮助你快速构建网页的基础样式，并且可以在编写自定义样式时作为参考。有时，开发者会使用CSS的reset或normalize方法来重置或统一不同浏览器的默认样式，以便更好地控制网页的整体外观和响应性。