1、margin
1.1 margin的语法
盒子与盒子之间的距离
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>div{background-color: pink;width: 100px;height: 100px;}.one{margin-bottom: 20px;}.two{margin-top: 20px;margin-bottom: 20px;margin-left: 20px;margin-right: 20px;}</style></head><body><div class="one"></div><div class="two"></div></body></html>
1.2 margin复合写法
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>div{background-color: pink;width: 100px;height: 100px;}.one{margin: 20px;}.two{margin-top: 20px;margin-bottom: 20px;margin-left: 20px;margin-right: 20px;}</style></head><body><div class="one"></div><div class="two"></div></body></html>
1.3 margin外边距典型应用
(1)使块级盒子水平居中
条件:
(1)盒子必须指定宽度(width)
(2)盒子左右外边距设置为auto
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>div{background-color: pink;width: 100px;height: 100px;}.one{margin: 0 auto;}</style></head><body><div class="one"></div><!-- <div class="two"></div> --></body></html>
对行内元素和行内块元素无效,但要实现水平居中的效果,可以其父元素添加text-align:center属性
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>div{background-color: pink;width: 200px;height: 200px;text-align: center;}.one{margin: 0 auto;}</style></head><body><div class="one"><span>行内元素的居中对齐</span></div><!-- <div class="two"></div> --></body></html>
1.4 外边距合并和塌陷
使用margin定义块元素的垂直外边距(top和bottom)时,可能会出现外边距的合并和塌陷
主要有两种情况:
(1)相邻块元素垂直外边距的合并(兄弟关系的)
(2)嵌套块元素垂直外边距的塌陷(父子关系的)
什么是外边距合并:
one盒子设置下边距为10,two盒子设置上边距为20.但实际两个盒子的距离不是10+20。而是取的两值间的较大值,这种现象就算相邻块元素垂直外边距的合并
** 浮动的盒子不会有外边距合并塌陷问题**
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>div{width: 200px;height: 200px;background-color: green;}.one{margin-bottom: 10px;}.two{margin-top: 20px;}</style></head><body><div class="one"></div><div class="two"></div></body></html>
解决方案:
尽量只给一个盒子添加margin值
什么是外边距的塌陷
对于嵌套关系(父子关系)的块元素,父元素有margin-top的同时,子元素也有margin-top,此时父元素会塌陷两者中较大的margin-top值
期望的效果:
实际的效果:
如何解决:
(1)父元素加上边框top(颜色可以设为transparent)–恩断义绝框,影响盒子大小
(2)父元素定义上内边距top。影响盒子大小
(3)给父元素加上overflow:hidden,不影响盒子大小
(4)其他方法:浮动、固定、绝对定位的盒子不会有塌陷问题
执行第(1)种:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>.farther{width: 200px;height: 200px;margin-top: 20px;border-top:2px solid black;background-color: green;}.son{width: 50px;height: 50px;margin-top: 50px;background-color: pink;}</style></head><body><div class="farther"><div class="son"></div></div></body></html>
执行第(2)种:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>.farther{width: 200px;height: 200px;margin-top: 20px;padding-top: 50px;background-color: green;}.son{width: 50px;height: 50px;margin-top:50px;background-color: pink;}</style></head><body><div class="farther"><div class="son"></div></div></body></html>
(3)执行第(3)种:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>.farther{width: 200px;height: 200px;margin-top: 20px;background-color: green;overflow: hidden;}.son{width: 50px;height: 50px;margin-top: 50px;background-color: pink;}</style></head><body><div class="farther"><div class="son"></div></div></body></html>
1.5 清除内外边距
不同的网页元素(如ul),即使没有设置内外边距,但有的自身默认会有内外边距(不同浏览器默认的也不一致),因此在布局前,首先最好先清除网页元素的内外边距
//可以将这段代码放在css的首行去写*{margin:0;padding:0;}
注意:
行内元素为了照顾兼容性,尽量只设置左右内外边距,不要设置上下内外边距,但是转换为块级和行内块元素就可以了
ul没有设置padding-left,但实际看浏览器是由一个40px的padding-left
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title></head><body><ul><li>aaa</li></ul></body></html>
清除自带的内外边距:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>*{margin: 0;padding: 0;}</style></head><body><ul><li>aaa</li></ul></body></html>
行内元素设置上下边距:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>*{margin: 0;padding: 0;}span{margin: 20px;}</style></head><body><ul><li>aaa</li></ul><span>span</span></body></html>