Semantics and screen readers

Have you ever stopped to wonder how assistive technology, such as a screen reader, knows what to announce to users? The answer is that these technologies rely on developers marking up their pages with semantic HTML. But what are semantics, and how do screen readers use them?

Affordances and semantics

Before diving into semantics, it's helpful to understand another term: affordances. An affordance is any object that offers, or affords, its user the opportunity to perform an action. A classic example is the teapot:

A teapot's handle is a natural affordance.

This teapot doesn't need an instruction manual; instead, its physical design conveys to the user how it should be operated. It has a handle, and because you've seen other objects in the world with similar handles, you can infer how you should pick it up and operate it.

When we build graphical user interfaces, we use things like CSS to add visual affordances to our UI. For instance, you might give a button a drop shadow and border so that it resembles an actual button in the real world.

But if a user is unable to see the screen, then these visual affordances will not be conveyed to them. Therefore, you need to make sure that your UI is constructed in a way that can convey these same affordances to assistive technology. This non-visual exposure of a UI element's affordances is called its semantics.

Use semantic HTML

The easiest way of conveying proper semantics is to use semantically rich HTML elements.

Using CSS, it's possible to style the <div> and <button> elements so they convey the same visual affordances, but the two experiences are very different when using a screen reader. A <div> is just a generic grouping element, so a screen reader only announces the text content of the <div>. The <button> is announced as a "button," a much stronger signal to the user that it's something they can interact with.

The simplest and often best solution to this problem is to avoid custom interactive controls altogether. For example, replace a <div> that's acting like a button with an actual <button>.

Semantic properties and the accessibility tree

Generally speaking, every HTML element will have some of the following semantic properties:

  • A role or type
  • A name
  • A value (optional)
  • A state (optional)

An element's role describes its type, for example, "button," "input," or even just "group" for things like div and span elements.

An element's name is its computed label. Screen readers typically announce an element's name followed by its role, e.g. "Sign Up, button." The algorithm that determines an element's name factors in things like if there is any text content inside the element, whether or not it has attributes such as title or placeholder, whether or not the element is associated with an actual <label> element, and if the element has any ARIA attributes such as aria-label and aria-labelledby.

Some elements may have a value. For instance, <input type="text"> may have a value that reflects whatever the user has typed into the text field.

Some elements may also have a state, which conveys their current status. For instance, a <select> element can be in either an expanded or a collapsed state, depending on if it's open or closed.

The accessibility tree

For each node in the DOM, the browser determines if the node is semantically "interesting" and adds it to the accessibility tree. When assistive technology, like a screen reader, is providing an alternative UI to the user, it is often doing so by walking this accessibility tree.

Using Chrome's DevTools you can inspect an element's semantic properties and explore its position in the accessibility tree.

Next steps

Once you know a bit about semantics and how they aid screen reader navigation, you can't help but look at the pages you build differently. In the next section, we'll take a step back and consider how the entire outline of a page can be conveyed using effective headings and landmarks.