What does @media screen and (max-width: 1024px) mean in CSS?

I found this piece of code in a CSS file I inherited, but I can't make any sense out of it:

@media screen and (max-width: 1024px){
    img.bg {
        left: 50%;
        margin-left: -512px; }
}

Specifically, what is happening on the first开发者_如何学JAVA line?

That’s a media query. It prevents the CSS inside it from being run unless the browser passes the tests it contains.

The tests in this media query are:

1. @media screen — The browser identifies itself as being in the “screen” category. This roughly means the browser considers itself desktop-class — as opposed to e.g. an older mobile phone browser (note that the iPhone, and other smartphone browsers, do identify themselves as being in the screen category), or a screenreader — and that it’s displaying the page on-screen, rather than printing it.

2. max-width: 1024px — the width of the browser window (including the scroll bar) is 1024 pixels or less. (CSS pixels, not device pixels.)

That second test suggests this is intended to limit the CSS to the iPad, iPhone, and similar devices (because some older browsers don’t support max-width in media queries, and a lot of desktop browsers are run wider than 1024 pixels).

However, it will also apply to desktop browser windows less than 1024 pixels wide, in browsers that support the max-width media query.

Here’s the Media Queries spec, it’s pretty readable:

• http://www.w3.org/TR/css3-mediaqueries/

It's limiting the styles defined there to the screen (e.g. not print or some other media) and is further limiting the scope to viewports which are 1024px or less in width.

http://www.css3.info/preview/media-queries/

It says: When the page render on the screen at a resolution of max 1024 pixels in width then apply the rule that follow.

As you may already know in fact you can target some CSS to a media type that can be one of handheld, screen, printer and so on.

Have a look here for details..

In my case I wanted to center my logo on a website when the browser has 800px or less, then I did this by using the @media tag:

@media screen and (max-width: 800px) {
  #logo {
    float: none;
    margin: 0;
    text-align: center;
    display: block;
    width: auto;
  }
}

It worked for me, hope somebody find this solution useful. :) For more information see this.

That's Media Queries. It allows you to apply part of CSS rules only to the specific devices on specific configuration.

It means if the screen size is 1024 then only apply below CSS rules.

If your media query condition is true then your CSS with that condition will work. That means CSS within your media query's condition pixel size will effect, or else if the condition will fail that mean if the device's width is greater than 1024px than your CSS will not work.Because your media query condition false.

max-width is your max CSS limit till that width.

It targets some specified feature to execute some other codes...

For example:

@media all and (max-width: 600px) {
  .navigation {
    -webkit-flex-flow: column wrap;
    flex-flow: column wrap;
    padding: 0;

  }

the above snippet say if the device that run this program have screen with 600px or less than 600px width, in this case our program must execute this part .

Also worth noting you can use 'em' as well as 'px' - blogs and text based sites do it because then the browser makes layout decisions more relative to the text content.

On Wordpress twentysixteen I wanted my tagline to display on mobiles as well as desktops, so I put this in my child theme style.css

@media screen and (max-width:59em){
    p.site-description {
        display:    block;
    }
}