Developers often ask how various CSS declarations translate to the screen reader experience. Properties that are strictly visualâ€”such as color, border, font, margin, paddingâ€”are transparent, but what about those that inject content, like
::after? What about properties that communicate meaning, like
line-through? And then there are those that visually position or crop content, like
visibility… the list goes on. We all know that using CSS to generate content is bad, just as we all know that we should never ever exceed the speed limit. It happens.
Earlier this year, three colleagues and I tested and documented the behavior of some widely-used CSS declarations in screen readers, leaving the screen reader user settings at their defaults. We reported our findings at AccessU 2017 and will present again, with some new material added, at Accessing Higher Ground.
In addition to myself, our team included CB Averitt and Steve Sawczyn of Deque, and Birkir Gunnarsson. We tested the following screen reader-browser pairings:
- JAWS/IE 11
- VoiceOver/Mobile Safari
- Talkback/Mobile Chrome
Different screen reader/browser pairings behave differently.
It’s tempting to assert that if you do x, â€œthe screen readerâ€ will announce y. Sometimes it really is just that simple, but in a surprising number of situations, it just isn’t that absolute. For example:
- Across the pairings we tested,
counterwas announced three different ways. (CSS counters are “variables” maintained by CSS whose values can be incremented by CSS rules, to track how many times they are used.)
vertical-align:superto communicate cents within a dollar amount worked in half of our pairings. In the other half, $1299 was announced as $1,299.
- Applying a transition to
opacity:0; visibility:hiddenon a paragraph, we logged five slightly different behaviors across the eight pairings.
DOM order is everything.
One of the consistencies we found is that, regardless of CSS position, content is read in the order that it appears in the DOM. For example, appending a
<div> to the end of the
<body> and then using
position:absolute to boost it to the top of the viewport will not change the reading order in the screen readerâ€”it will still come last (and in this case, we can safely say â€œtheâ€ screen reader).
The same is true for floats. Applying
float:right to an element typically positions it â€œafterâ€ (to the right of) the element that follows it in the DOM, making the visual reading order the opposite of the DOM order. Since the DOM order is what determines the screen reader reading order (and the tab order, in the case of active elements), the elements will be announced in the opposite order from the visual reading order.
Containers are only visual.
Many CSS properties are available to apply dimensions to containersâ€”
text-overflow determine whether and how content exceeding the bounds of the container are rendered. Across all of our test pairings, all of these are transparent to screen readers. Regardless of the content being clipped out visually, or obscured by
overflow:hidden, the screen reader will announce all content in the container. Even
opacity:0 has no effect in the screen reader environment; the content is announced nonetheless.
The moral of the story
All of this really highlighted the value of accessibility-minded development and pre-launch testingâ€”in a wide variety of browser-screen reader combinationsâ€”to ensure that all website visitors enjoy a consistent, equivalent experience.