I have heard a lot of my friends talk about using w开发者_如何学Pythonrappers in CSS to center the "main" part of a website.

Is this the best way to accomplish this? What is best practice? Are there other ways?

Most basic example (live example here):

CSS:

    #wrapper {
        width: 500px;
        margin: 0 auto;
    }

HTML:

    <body>
        <div id="wrapper">
            Piece of text inside a 500px width div centered on the page
        </div>
    </body>

How the principle works:

Create your wrapper and assign it a certain width. Then apply an automatic horizontal margin to it by using margin: 0 auto; or margin-left: auto; margin-right: auto;. The automatic margins make sure your element is centered.

The best way to do it depends on your specific use-case.

However, if we speak for the general best practices for implementing a CSS Wrapper, here is my proposal: introduce an additional <div> element with the following class:

/**
 * 1. Center the content. Yes, that's a bit opinionated.
 * 2. Use `max-width` instead `width`
 * 3. Add padding on the sides.
 */
.wrapper {
    margin-right: auto; /* 1 */
    margin-left:  auto; /* 1 */

    max-width: 960px; /* 2 */

    padding-right: 10px; /* 3 */
    padding-left:  10px; /* 3 */
}

... for those of you, who want to understand why, here are the 4 big reasons I see:

1. Use `max-width` instead `width`

In the answer currently accepted Aron says width. I disagree and I propose max-width instead.

Setting the width of a block-level element will prevent it from stretching out to the edges of its container. Therefore, the Wrapper element will take up the specified width. The problem occurs when the browser window is smaller than the width of the element. The browser then adds a horizontal scrollbar to the page.

Using max-width instead, in this situation, will improve the browser's handling of small windows. This is important when making a site usable on small devices. Here’s a good example showcasing the problem:

/**
 * The problem with this one occurs
 * when the browser window is smaller than 960px.
 * The browser then adds a horizontal scrollbar to the page.
 */
.width {
    width: 960px;
    margin-left: auto;
    margin-right: auto;
    border: 3px solid #73AD21;
}

/**
 * Using max-width instead, in this situation,
 * will improve the browser's handling of small windows.
 * This is important when making a site usable on small devices.
 */
.max-width {
    max-width: 960px;
    margin-left: auto;
    margin-right: auto;
    border: 3px solid #73AD21;
}

/**
 * Credits for the tip: W3Schools
 * https://www.w3schools.com/css/css_max-width.asp
 */

<div class="width">This div element has width: 960px;</div>
<br />

<div class="max-width">This div element has max-width: 960px;</div>

So in terms of Responsiveness, is seems like max-width is the better choice!-

2. Add Padding on the Sides

I’ve seen a lot of developers still forget one edge case. Let’s say we have a Wrapper with max-width set to 980px. The edge case appears when the user’s device screen width is exactly 980px. The content then will exactly glue to the edges of the screen with not any breathing space left.

Generally, we’d want to have a bit of padding on the sides. That’s why if I need to implement a Wrapper with a total width of 980px, I’d do it like so:

.wrapper {
   max-width: 960px; /** 20px smaller, to fit the paddings on the sides */
   
   padding-right: 10px;
   padding-left:  10px;
   
   /** ... omitted for brevity */
}

Therefore, that’s why adding padding-left and padding-right to your Wrapper might be a good idea, especially on mobile.

Or, consider using box-sizing so that the padding doesn’t change the overall width at all.

3. Use a `<div>` Instead of a `<section>`

By definition, the Wrapper has no semantic meaning. It simply holds all visual elements and content on the page. It’s just a generic container. Therefore, in terms of semantics, <div> is the best choice.

One might wonder if maybe a <section> element could fit this purpose. However, here’s what the W3C spec says:

The element is not a generic container element. When an element is needed only for styling purposes or as a convenience for scripting, authors are encouraged to use the div element instead. A general rule is that the section element is appropriate only if the element's contents would be listed explicitly in the document's outline.

The <section> element carries it’s own semantics. It represents a thematic grouping of content. The theme of each section should be identified, typically by including a heading (h1-h6 element) as a child of the section element.

Examples of sections would be chapters, the various tabbed pages in a tabbed dialog box, or the numbered sections of a thesis. A Web site's home page could be split into sections for an introduction, news items, and contact information.

It might not seem very obvious at first sight, but yes! The plain old <div> fits best for a Wrapper!

4. Using the `<body>` Tag vs. Using an Additional `<div>`

Here's a related question. Yes, there are some instances where you could simply use the <body> element as a wrapper. However, I wouldn’t recommend you to do so, simply due to flexibility and resilience to changes.

Here's an use-case that illustrates a possible issue: Imagine if on a later stage of the project you need to enforce a footer to "stick" to the end of the document (bottom of the viewport when the document is short). Even if you can use the most modern way to do it - with Flexbox, I guess you need an additional Wrapper <div>.

I would conclude it is still best practice to have an additional <div> for implementing a CSS Wrapper. This way if spec requirements change later on you don't have to add the Wrapper later and deal with moving the styles around a lot. After all, we're only talking about 1 extra DOM element.

You don't need a wrapper, just use the body as the wrapper.

CSS:

body {
    margin:0 auto;
    width:200px;
}

HTML:

<body>
    <p>some content</p>
<body>

<div class="wrapper">test test test</div>

.wrapper{
    width:100px;
    height:100px;
    margin:0 auto;
}

Check working example at http://jsfiddle.net/8wpYV/

The easiest way is to have a "wrapper" div element with a width set, and a left and right margin of auto.

Sample markup:

<!doctype html>
<html>
<head>
    <title></title>
    <style type="text/css">
        .wrapper { width: 960px; margin: 0 auto; background-color: #cccccc; }
        body { margin: 0; padding: 0 }
    </style>
</head>
<body>
    <div class="wrapper">
        your content...
    </div>
</body>
</html>

a "wrapper" is just a term for some element that encapsulates all other visual elements on the page. The body tag seems to fit the bill, but you would be at the mercy of the browser to determine what displays beneath that if you adjust the max-width.

Instead, we use div because it acts as a simple container that does not break. the main, header, footer, and section tags in HTML5 are just div elements named appropriately. It seems that there could (or should) be a wrapper tag because of this trend, but you may use whichever method of wrapping you find most suitable for your situation. through classes, ids and css, you can use a span tag in a very similar way.

There are a lot of HTML element tags that we do not use often or possibly even know about. Doing some research would show you what can be done with pure HTML.

Are there other ways?

Negative margins were also used for horizontal (and vertical!) centering but there are quite a few drawbacks when you resize the window browser: no window slider; the content can't be seen anymore if the size of the window browser is too small.
No surprise as it uses absolute positioning, a beast never completely tamed!

Example: http://bluerobot.com/web/css/center2.html

So that was only FYI as you asked for it, margin: 0 auto; is a better solution.

Centering content has so many avenues that it can't really be explored in a single answer. If you would like to explore them, CSS Zen Garden is an enjoyable-if-old resource exploring the many, many ways to layout content in a way even old browsers will tolerate.

The correct way, if you don't have any mitigating requirements, is to just apply margin: auto to the sides, and a width. If your page has no content that needs to go outside those margins, just apply it to the body:

body {
  padding: 0;
  margin: 15px auto;
  width: 500px;
}

https://jsfiddle.net/b9chris/62wgq8nk/

So here we've got a 500px wide set of content centered at all* sizes. The padding 0 is to deal with some browsers that like to apply some default padding and throw us off a bit. In the example I do wrap the content in an article tag to be nice to Screen Readers, Pocket, etc so for example the blind can jump past the nav you likely have (which should be in nav) and straight to the content.

I say all* because below 500px this will mess up - we're not being Responsive. To get Responsive, you could just use Bootstrap etc, but building it yourself you use a Media Query like:

body {
  padding: 0;

  margin: 15px;
  @media (min-width: 500px) {
    margin: 15px auto;
    width: 500px;
  }
}

Note that this is SCSS/SASS syntax - if you're using plain CSS, it's inverted:

body {
  padding: 0;  
  margin: 15px;
}

@media (min-width: 500px) {
  body {
    margin: 15px auto;
    width: 500px;
  }
}

https://jsfiddle.net/b9chris/62wgq8nk/6/

It's common however to want to center just one chunk of a page, so let's apply this to only the article tag in a final example.

body {
  padding: 0;
  margin: 0;
}

nav {
  width: 100%;
  box-sizing: border-box;
  padding: 15px;
}

article {
  margin: 15px;
  @media (min-width: 500px) {
    margin: 15px auto;
    width: 500px;
  }
}

https://jsfiddle.net/b9chris/62wgq8nk/17/

Note that this final example also uses CSS Flexbox in the nav, which is also one of the newer ways you could center things. So, that's fun.

But, there are special circumstances where you need to use other approaches to center content, and each of those is probably worth its own question (many of them already asked and answered here on this site).

/******************
Fit the body to the edges of the screen
******************/    

body {
         margin:0;
         padding:0;
    }

header {
     background:black;
     width:100%;
}

.header {
     height:200px;
}

nav {
     width:100%;
     background:lightseagreen;
}

.nav {
     padding:0;
     margin:0;
}

.nav a {
     padding:10px;
     font-family:tahoma;
     font-size:12pt;
     color:white;
}

/******************
Centered wrapper, all other content divs will go inside this and will never exceed the width of 960px.
******************/

    .wrapper {
         width:960px;
         max-width:100%;
         margin:0 auto;
    }



<!-------- Start HTML ---------->

    <body>

<header>

    <div id="header" class="wrapper">

    </div>

</header>

<nav>

     <div id="nav" class="wrapper">

     </div>

</nav>



    </body>