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css常考面试题

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1.盒模型宽度计算

2.margin纵向重叠问题

3.margin负值问题

4.bfc理解和应用

5.float布局

6.flex布局:实现一个三点的色子

7.css定位

8.line-height如何继承

9.css响应式

下面依次看下:

1.盒模型宽度计算,下面div的offsetWidth

<style type="text/css">        #div1 {            width: 100px;            padding: 10px;            border: 1px solid #ccc;            margin: 10px;        }</style> <div id="div1">        this is div1 </div>

offsetWidth = 内容宽度 + 内边距 + 边框。无外边距。

所以 offsetWidth = 100 + 10 * 2 + 1 * 2 = 122px

补充:如果让offsetWidth = 100px ,可以怎么做

答:加上box-sizing: border-box;

2.margin纵向重叠问题:如下代码,AAA和BBB之间的距离是多少?

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>margin 纵向重叠</title>    <style type="text/css">        p {            font-size: 16px;            line-height: 1;            margin-top: 10px;            margin-bottom: 15px;        }</style></head><body>        <p>AAA</p>    <p></p>    <p></p>    <p></p>    <p>BBB</p></body></html>

知识点:

1.相邻元素的margin-top和margin-bottom 会发生重叠

2.空白内容的p也会重叠

3.所以答案是:15px

3.margin负值问题

知识点:

1.margin-top和margin-left负值,元素向上,向左移动

2.margin-right负值,右侧元素左移,自身不受影响

3.margin-bottom负值,下方元素上移,自身不受影响

4.bfc理解和应用

概念:

1.块级格式化上下文

2.一块独立渲染区域,内部元素的渲染不会影响边界以外的元素

形成bfc的常见条件:

1.float不是none

2.position是absolute或fixed

3.overflow不是visible

4.display是flex inline-block等

bfc常见应用:清除浮动

// bfc前<style type="text/css">        .container {            background-color: #f1f1f1;        }        .left {            float: left;        }</style><div class="container">        <img src="; class="left" style="magin-right: 10px;"/>        <p>某一段文字……</p></div>// bfc后:在父元素和p元素上添加bfc<style type="text/css">        .container {            background-color: #f1f1f1;        }        .left {            float: left;        }        .bfc {            overflow: hidden; /* 触发元素 BFC */        }</style><div class="container bfc">        <img src="; class="left" style="magin-right: 10px;"/>        <p class="bfc">某一段文字……</p></div>

5.float布局

知识点:

1.如何实现圣杯布局和双飞翼布局

2.手写clearfix

圣杯布局和双飞翼布局的目的:

1.三栏布局,中间一栏最先加载和渲染

2.两侧内容固定,中间内容随着宽度自适应

圣杯布局和双飞翼布局的目的:

1.使用float布局

2.两侧使用margin负值,以便和中间内容横向重叠

3.防止中间内容被两侧覆盖,一个用padding,一个用margin

圣杯布局:

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>圣杯布局</title>    <style type="text/css">        body {            min-width: 550px;        }        #header {            text-align: center;            background-color: #f1f1f1;        }        #container {            padding-left: 200px;            padding-right: 150px;        }        #container .column {            float: left;        }        #center {            background-color: #ccc;            width: 100%;        }        #left {            position: relative;            background-color: yellow;            width: 200px;            margin-left: -100%;            right: 200px;        }        #right {            background-color: red;            width: 150px;            margin-right: -150px;        }        #footer {            text-align: center;            background-color: #f1f1f1;        }        /* 手写 clearfix */        .clearfix:after {            content: '';            display: table;            clear: both;        }</style></head><body>    <div id="header">this is header</div>    <div id="container" class="clearfix">        <div id="center" class="column">this is center</div>        <div id="left" class="column">this is left</div>        <div id="right" class="column">this is right</div>    </div>    <div id="footer">this is footer</div></body></html>

双飞翼布局:

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>双飞翼布局</title>    <style type="text/css">        body {            min-width: 550px;        }        .col {            float: left;        }        #main {            width: 100%;            height: 200px;            background-color: #ccc;        }        #main-wrap {            margin: 0 190px 0 190px;        }        #left {            width: 190px;            height: 200px;            background-color: #0000FF;            margin-left: -100%;        }        #right {            width: 190px;            height: 200px;            background-color: #FF0000;            margin-left: -190px;        }</style></head><body>    <div id="main" class="col">        <div id="main-wrap">            this is main        </div>    </div>    <div id="left" class="col">        this is left    </div>    <div id="right" class="col">        this is right    </div></body></html>

6.flex布局:实现一个三点的色子

常用语法:

flex-direction justfy-content align-items flex-wrap align-self

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>flex 画骰子</title>    <style type="text/css">        .box {            width: 200px;            height: 200px;            border: 2px solid #ccc;            border-radius: 10px;            padding: 20px;            display: flex;            justify-content: space-between;        }        .item {            display: block;            width: 40px;            height: 40px;            border-radius: 50%;            background-color: #666;        }        .item:nth-child(2) {            align-self: center;        }        .item:nth-child(3) {            align-self: flex-end;        }</style></head><body>    <div class="box">        <span class="item"></span>        <span class="item"></span>        <span class="item"></span>    </div></body></html>

7.css定位

absolute和relative定位:

1.relative根据自身定位

2.absolute根据最近一层的定位元素定位,,找不到,就根据body定位

水平居中:

inline元素:text-aligin:center

block元素:margin: auto

absoulte元素:left:50% + margin-left负值

垂直居中:

inline元素:line-height的值等于height的值

absolute元素:top:50% + margin-top 负值。需要知道元素的宽高

absolute元素:transform(-50%, -50%) 不需要知道宽高

absolute元素:top,left,bottom,right = 0 + margin: auto 不需要知道宽高

水平对齐演示:

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>水平对齐</title>    <style type="text/css">        .container {            border: 1px solid #ccc;            margin: 10px;            padding: 10px;        }        .item {            background-color: #ccc;        }        .container-1 {            text-align: center;        }        .container-2 .item {            width: 500px;            margin: auto;        }        .container-3 {            position: relative;            height: 100px;        }        .container-3 .item {            width: 300px;            height: 100px;            position: absolute;            left: 50%;            margin-left: -150px;        }</style></head><body>    <div class="container container-1">        <span>一段文字</span>    </div>    <div class="container container-2">        <div class="item">            this is block item        </div>    </div>    <div class="container container-3">        <div class="item">            this is absolute item        </div>    </div></body></html>

垂直对齐演示:

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>垂直对齐</title>    <style type="text/css">        .container {            border: 1px solid #ccc;            margin: 10px;            padding: 10px;            height: 200px;        }        .item {            background-color: #ccc;        }        .container-1{            text-align: center;            line-height: 200px;            height: 200px;        }        .container-2 {            position: relative;        }        .container-2 .item {            width: 300px;            height: 100px;            position: absolute;            left: 50%;            margin-left: -150px;            top: 50%;            margin-top: -50px;        }        .container-3 {            position: relative;        }        .container-3 .item {            width: 200px;            height: 80px;            position: absolute;            left: 50%;            top: 50%;            transform: translate(-50%, -50%)        }        .container-4 {            position: relative;        }        .container-4 .item {            width: 100px;            height: 50px;            position: absolute;            top: 0;            left: 0;            bottom: 0;            right: 0;            margin: auto;        }</style></head><body>    <div class="container container-1">        <span>一段文字</span>    </div>    <div class="container container-2">        <div class="item">            this is item        </div>    </div>    <div class="container container-3">        <div class="item">            this is item        </div>    </div>    <div class="container container-4">        <div class="item">            this is item        </div>    </div></body></html>

8.line-height如何继承? 如下代码:p的行高是多少?

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>line-height 继承问题</title>    <style type="text/css">        body {            font-size: 20px;            line-height: 200%;        }        p {            background-color: #ccc;            font-size: 16px;        }</style></head><body>    <p>这是一行文字</p></body></html>

行高是40px。

有三种情况:

// body的line-height直接是数字,则p的line-height直接就是body的line-height,为40px<style type="text/css">        body {            font-size: 20px;            line-height: 40px;        }        p {            background-color: #ccc;            font-size: 16px;        }</style>// body的line-height直接是比例,则p的line-height直接就是p的font-size * 比例,为24px<style type="text/css">        body {            font-size: 20px;            line-height: 1.5;        }        p {            background-color: #ccc;            font-size: 16px;        }</style>// body的line-height直接是百分比,则p的line-height直接就是body的font-size * body的line-height,为40px<style type="text/css">        body {            font-size: 20px;            line-height: 200%;        }        p {            background-color: #ccc;            font-size: 16px;        }</style>

9.css响应式

1.rem是什么?

2.响应式布局方案?

rem是什么?

px:绝对长度单位,最常用

em:相对长度单位,相对于父元素,不常用

rem:相对长单位,相对于根元素html的font-size,用于响应式

代码演示:

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <meta http-equiv="X-UA-Compatible" content="ie=edge">    <title>rem 演示</title>    <style type="text/css">        html {            font-size: 100px; // 1rem = 100px        }        div {            background-color: #ccc;            margin-top: 10px;            font-size: 0.16rem;        }</style></head><body>    <p style="font-size: 0.1rem">rem 1</p>    <p style="font-size: 0.2rem">rem 1</p>    <p style="font-size: 0.3rem">rem 1</p>    <div style="width: 1rem;">        this is div1    </div>    <div style="width: 2rem;">        this is div2    </div>    <div style="width: 3rem;">        this is div3    </div></body></html>

响应式布局方案?

1.媒体查询

2.rem

3.vw/vh

vw:网页视口宽度的1/100

vh:网页视口高度的1/100

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