What is sr-only in Bootstrap 3?

According to bootstrap's documentation, the class is used to hide information intended only for screen readers from the layout of the rendered page.

Screen readers will have trouble with your forms if you don't include a label for every input. For these inline forms, you can hide the labels using the .sr-only class.

Here is an example styling used:

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  border: 0;
}

Is it important or can I remove it? Works fine without.

It's important, don't remove it.

You should always consider screen readers for accessibility purposes. Usage of the class will hide the element anyways, therefore you shouldn't see a visual difference.

If you're interested in reading about accessibility:

Web Accessibility Initiative (WAI)

MDN Accessibility documentation

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As JoshC said, the class .sr-only is used to visually hide the information used for screen readers only. But not only to hide labels. You might consider hiding various other elements such as "skip to main content" link, icons which have an alternative texts etc.

BTW. you can also use .sr-only sr-only-focusable if you need the element to become visible when focused e.g. "skip to main content"

If you want make your website even more accessible I recommend to start here:

Accessibility @Google - Web Fundamentals

Accessibility Developer Guide (my personal favorite)

WebAIM Principles + WebAIM WCAG Checklist

Accessibility @ReactJS (lots of good resources and general stuff)

Why?

According to the World Health Organization, 285 million people have vision impairments. So making a website accessible is important.

IMPORTANT: Avoid treating disabled users differently. Generally speaking try to avoid developing a different content for different groups of users. Instead try to make accessible the existing content so that it simply works out-of-the-box and for all not specifically targeting e.g. screen readers. In other words don't try to reinvent the wheel. Otherwise the resulting accessibility will often be worse than if there was nothing developed at all. We developers should not assume how those users will use our website. So be very careful when you need to develop such solutions. Obviously a "skip link" is a good example of such content if it's made visible when focused. But there many bad examples too. Such would be hiding from a screen reader a "zoom" button on the map assuming that it has no relevance to blind users. But surprisingly, a zoom function indeed is used among blind users! They like to download images like many other users do (even in high resolution), for sending them to somebody else or for using them in some other context. Source - Read more @ADG: Bad ARIA practices

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I found this in the navbar example, and simplified it.

<ul class="nav">
  <li><a>Default</a></li>
  <li><a>Static top</a></li>
  <li><b><a>Fixed top <span class="sr-only">(current)</span></a></b></li>
</ul>

You see which one is selected (sr-only part is hidden):

Default

Static top

Fixed top

You hear which one is selected if you use screen reader:

Default

Static top

Fixed top (current)

As a result of this technique blind people supposed to navigate easier on your website.

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.sr-only is a class name specifically used for screen readers. You can use any class name, but .sr-only is pretty commonly used. If you don't care about developing with compliance in mind, then it can be removed. It will not affect UI in any way if removed because the CSS for this class is not visible to desktop and mobile device browsers.

There seems to be some information missing here about the use of .sr-only to explain its purpose and being for screen readers. First and foremost, it is very important to always keep impaired users in mind. Impairment is the purpose of 508 compliance: https://www.section508.gov/, and it is great that bootstrap takes this into consideration. However, the use of .sr-only is not all that needs to be taken into consideration for 508 compliance. You have the use of color, size of fonts, accessibility via navigation, descriptors, use of aria and so much more.

But as for .sr-only - what does the CSS actually do? There are several slightly different variants of the CSS used for .sr-only. One of the few I use is below:

.sr-only {
    position: absolute;
    margin: -1px 0 0 -1px;
    padding: 0;
    display: block;
    width: 1px;
    height: 1px;
    font-size: 1px;
    line-height: 1px;
    overflow: hidden;
    clip: rect(0,0,0,0);
    border: 0;
    outline: 0;
    }

The above CSS hides content in desktop and mobile browsers wrapped with this class, but is seen by a screen reader like JAWS: http://www.freedomscientific.com/Products/Blindness/JAWS. Example markup is as follows:

<a href="#" target="_blank">
    Click to Open Site
    <span class="sr-only">This is an external link</span>
</a>

Additionally, if a DOM element has a width and height of 0, the element is not seen by the DOM. This is why the above CSS uses width: 1px; height: 1px;. By using display: none and setting your CSS to height: 0 and width: 0, the element is not seen by the DOM and is thus problematic. The above CSS using width: 1px; height: 1px; is not all you do to make the content invisible to desktop and mobile browsers (without overflow: hidden, your content would still show on the screen), and visible to screen readers. Hiding the content from desktop and mobile browsers is done by adding an offset from width: 1px and height: 1px previously mentioned by using:

position: absolute;
margin: -1px 0 0 -1px; 
overflow: hidden;

Lastly, to have a very good idea of what a screen reader sees and relays to its impaired user, turn off page styling for your browser. For Firefox, you can do this by going to:

View > Page Style > No Style

I hope the information I provided here is of further use to someone in addition to the other responses.

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Ensures that the object is displayed (or should be) only to readers and similar devices. It give more sense in context with other element with attribute aria-hidden="true".

<div class="alert alert-danger" role="alert">
  <span class="glyphicon glyphicon-exclamation-sign" aria-hidden="true"></span>
  <span class="sr-only">Error:</span>
  Enter a valid email address
</div>

Glyphicon will be displayed on all other devices, word Error: on text readers.

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The .sr-only class hides an element to all devices except screen readers:

Skip to main content Combine .sr-only with .sr-only-focusable to show the element again when it is focused

.sr-only {
  border: 0 !important;
  clip: rect(1px, 1px, 1px, 1px) !important; /* 1 */
  -webkit-clip-path: inset(50%) !important;
  clip-path: inset(50%) !important; /* 2 */
  height: 1px !important;
  margin: -1px !important;
  overflow: hidden !important;
  padding: 0 !important;
  position: absolute !important;
  width: 1px !important;
  white-space: nowrap !important; /* 3 */
}

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