Whilst audio and video is an important aspect of the Web, not everyone can experience it. For example, people who are partially-sighted may not be able to see videos very well and people who are blind may not be able to see them at all. Some users may disable plug-ins for audio and video content (e.g. Flash or Quicktime) to reduce bandwidth or they may not be able to use them at all on mobile devices. Also. search engines, such as Google, will only index the text on a webpage, therefore any information conveyed through non-text content will not be processed. To address this, it is important that all audio and video content has a text alternative that adequately describes its purpose.
Alternative text acts as a fallback for people who, for whatever reason, cannot see audio and video content. It means that people who are blind or partially-sighted can use assistive technology, such as a screen reader or screen magnifier, to read the text alternative of audio and video content in much the same way they would read other text content on a webpage. Users who have disabled plug-ins for audio and video content or who are unable to see it on their mobile device will see the text alternative instead, and search engines will be able to process the text alternative and properly index the webpage.
There are various methods of providing alternative text.
To add alternative text to an object element (which is typically used to embed audio and video content, such as Flash or Java), include a paragraph element <p> containing the alternative text within the object element.
Flash, Java and other plug-ins also have their own accessibility options to allow you to specify alternative text descriptions.
HTML5 also has new native <audio> and <video> elements that can make the creation of accessible players much easier. WAI-ARIA also provides ways to describe the purpose of audio and video content by using the "aria-labelledby" by or "aria-describedby" attributes, provided there is an existing inpage description.
//A paragraph element is included within the object element to provide an alternative text description.
<object type="application/x-shockwave-flash" width="600" height="400">
<param name="movie" value="movies/.swf" />
<p>A video of the first manned lunar landings in 1969.<p>
</object>