Take a look at these front-end interview questions to help you master high-frequency knowledge points (3)

10 questions every day, after 100 days, you will have mastered all the high-frequency knowledge points of front-end interviews, come on! ! ! , while reading the article, I hope you don’t look at the answer directly, but first think about whether you know it, and if so, what is your answer? Think about it and then compare it with the answer. Would it be better? Of course, if you have a better answer than mine, please leave a message in the comment area and discuss the beauty of technology together.

Interviewer: What are the ways to clear floats?

Me: Uh~, floating elements will break away from the document flow (absolutely positioned elements will also break away from the document flow), resulting in the inability to calculate the accurate height. This problem is called: "High collapse”.

There are three common ways to clear floats. The entire code is as follows:

Trigger BFC: (It is defective and will Causes the content to overflow and hide)

<style>
    *{margin: 0;padding: 0;}
    ul {
        list-style: none;
        border: 10px solid #ccc;
        overflow: hidden; /*触发BFC清除浮动*/
    }
    ul li {
        width: 100px;
        height: 100px;
        background-color: #f00;
        float: left;
    }
</style>

    

        
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Create one more box and add the style: clear:both; : (not recommended, this method is obsolete) [ Related recommendations: web front-end development】

<style>
    *{margin: 0;padding: 0;}
    ul {
        list-style: none;
        border: 10px solid #ccc;
    }
    ul li {
        width: 100px;
        height: 100px;
        background-color: #f00;
        float: left;
    }
    ul div{
        clear: both;
    }
</style>

    

        
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Add an after style to the floating parent element: (recommended)

<style>
    *{margin: 0;padding: 0;}
    ul {
        list-style: none;
        border: 10px solid #ccc;
    }
    ul li {
        width: 100px;
        height: 100px;
        background-color: #f00;
        float: left;
    }
    ul::after{
        content: &#39;&#39;;
        display: block;
        clear: both;
    }
</style>

    

        
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Interviewer: Should you use odd or even numbers of fonts in web pages?

Me: Uh~, you should use even numbers, because even numbers can make the text perform better on the browser, and the UI design drawings for the front end are generally even numbers, so Whether it’s layout or px conversion, it’s more convenient.

Interviewer: What are the values ​​of position? What positioning are they based on?

Me: Uh~, position has the following five values:

static: default value, no positioning, top, right, bottom, left have no effect

relative: relative positioning, does not break away from the document flow, only left and top work

absolute: absolute positioning, breaks away from the document flow, the top, bottom, left, and right are based on the nearest positioned parent element as the reference system

fixed: Break away from the document flow, use the browser viewport as the reference system for top, bottom, left and right

sticky: a combination of relative and fixed

Take fixed as an example:

<style>
    *{margin: 0;padding: 0;}
    body{
        height: 2000px;
    }
    .main{
        width: 100px;
        height: 100px;
        left: 20px;
        top: 50px;
        background-color: #f00;
        position: fixed;
    }
</style>

    <div>221</div>

Interviewer: Write a left, middle and right layout that fills the screen. The left and right blocks have a fixed width of 200px and the middle block is adaptive. The middle block is required to be loaded first. Please write down the structure. and its style.

Me: Uh~, okay, the whole code is as follows:

<style>
    *{margin: 0;padding: 0;}
    body{width: 100vw;height: 100vh;}
    .container{
        height: 100%;
        width: 100%;
    }
    .container>div{
        float: left;
    }
    .zhong{
        height: 100vh;
        width: 100%;
        background-color: pink;
    }
    .zhong .main{
        margin: 0 200px;
    }
    .zuo{
        width: 200px;
        height: 100vh;
        background-color: #f00;
        margin-left: -100%;
    }
    .you{
        width: 200px;
        height: 100vh;
        background-color: #0f0;
        margin-left: -200px;
    }
</style>


    <div>
        <div>
            <div>中</div>
        </div>
        <div>左</div>
        <div>右</div>
    </div>

Interviewer: What is CSS reset?

Me: Uh~, some CSS tags have specific styles by default, and we need to remove them because we don’t need these styles.

reset.css is a CSS file used to reset CSS styles. The official website is: resetcss

Normalize.css is a CSS style reset table to enhance cross-browser rendering consistency. Official website: Normalize.css

The difference between the two:

normalize.css: Useful styles will be retained. For example, the font size of h1

reset.css: resets all styles. For example, the font size of h1, h2, and h3 is reset and remains unstyled

If it is normal project, I personally prefer reset.css.

面试官：display: none; 与 visibility: hidden; 的区别？

我：呃~，display: none; ：隐藏元素但不占用位置。visibility: hidden; ：隐藏元素但占用位置

display: none; 和 visibility: hidden; 都会造成页面重绘，使得页面样式改变，但是display: none; 还会产生一次回流，改变了元素的位置。

面试官：opacity 和 rgba 的区别

共同性：实现透明效果。

1. opacity：取值范围0到1之间，0表示完全透明，1表示不透明
2. rgba：R表示红色，G表示绿色，B表示蓝色，取值可以在正整数或者百分数。A表示透明度取值0到1之间。
两者的区别：

继承的区别，opacity会继承父元素的opacity属性，而RGBA设置的元素的后代元素不会继承不透明属性。整出代码如下：

<style>
    .opacity{
        width: 100%;
        height: 200px;
        font-size: 50px;
        font-weight: bold;
        background-color: #f00;
        opacity: 0.5;
    }
    .rgba{
        width: 100%;
        height: 200px;
        font-size: 50px;
        font-weight: bold;
        background-color: #f00;
        background: rgba(255, 0, 0, .5);
    }
</style>

    <div>这是opacity</div>
    <hr>
    <div>这是rgba</div>

面试官：伪类与伪元素有什么区别？解释一下伪元素的作用

我：呃~，好的，两者的区别如下：

伪类使用单冒号，而伪元素使用双冒号。如 :hover 是伪类，::before 是伪元素

伪元素会在文档流生成一个新的元素，并且可以使用 content 属性设置内容

面试官：rem、em、vw、vh 的值各是什么意思？

我：呃~，好的，他们各值的意思如下：

rem：根据根元素(即 html)的 font-size

em：根据自身元素的 font-size

vw：viewport width

vh：viewport height

面试官：webkit表单输入框placeholder的颜色值能改变吗?

我：呃~，是可以修改的，整出代码如下：

<style>
    input::-webkit-input-placeholder{
        color: blue;
    }
</style> 

    <input>

（学习视频分享：web前端入门、编程基础视频）

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