SVG has a few metadata features intended specifically for accessibility, and also provides the ability to use real text instead of images for textual content, just like HTML. This combination of text and metadata serves as one of the cornerstones of SVG’s accessibility; there are other features, such as scalability and navigation features, but this post will focus on the descriptive capabilities of SVG.
I was recently looped into a discussion on the @longdesc attribute in HTML which dealt in part with SVG accessibility, a subject I’m fascinated by. The specific debate is whether a @longdesc value should be applied for SVG, or whether SVG’s native accessibility features should be used, or both.
Let me explain @longdesc. For short descriptions of a picture, a few words to a few sentences, you can simply include the text in the <img> element’s @alt attribute. The @longdesc attribute of an <img> element allows a content author to add a URL that leads to a longer text description of the image. You can read more in this excellent article on @longdesc by WebAIM.
SVG provides a different way to provide text descriptions: the <title> and <desc> elements, which can be a child of any graphic or container element in SVG, and which contain text descriptions of the element. The <title> is meant for shortish names, while the <desc> can provide arbitrarily long descriptions; nothing in the SVG spec limits the length of these elements, but choosing the appropriate text is a best practice (as for any prose). These metadata elements can be read out via screenreaders (accessibility technology that speaks the available text aloud), along with the content of the <text>, <tspan>, and <textPath> elements.
Because each SVG shape (or group of shapes) can have its own <title> and <desc>, a certain amount of structure and sequential flow is available to these screen readers. Each text or metadata element is read out in document order. Together, the series of text passages can comprise a complete description of the SVG graphic. Which is kinda cool… self-describing images!
Well, that’s the idea, anyway. Now let’s look at it in practice, and specifically, at authoring SVG accessible content, and support in browsers and screenreaders.
Note: though I offer some specific guidance here, this is not really a tutorial; it’s more a background article on the topic, partly from a standardization point of view. I’ll be writing a tutorial aimed at developers and designers soon on WebPlatform.org.