Accessibility and Unicode
Here at Penn State my duties include being an accessibility guru as well as being a Unicode guru, and not too surprisingly, Unicode can enhance accessibility in some situations. And not just in the abstract "standards enhance accessibility" but more concretely as in:
It's An Encoded Character, Not a Font Trick
We all know that relying on fonts to display characters (e.g. the use of the Symbol font for Greek characters) is a Bad, Bad Idea, but it's even worse for a screen reader. Consider the expression θ = 2π. In the old Symbol font days, this might have been coded as:
<p> <font face="Symbol">q = 2p</font></p>
And guess what the screen reader would read - Q equals 2 P. Since the screen reader is essentially "font blind", the underlying text is what is read. Hence the Unicode correct code below is preferred:
<p> θ = 2π</p>-OR-
<p> θ = 2π</p>
If you think about it, the screen reader is a good tool for conceptualizing how characters (and their variants) may function semantically in different contexts.
I should mention that screen readers can get confused with a Unicode character if it can't recognize it, but that's more of a dictionary problem than a Unicode problem. For Jaws, it is possible to install .sbl pronunciation files to increase the character repertoire, especially for math and science.
It's Text, Not An Image
Perhaps the biggest advantage for Unicode though is that it allows characters that used to be embedded in images to be just plain text. For instance you could embed the following equation for the volume of a sphere:
V = 4/3πr³
Consider what happens though if a low-vision reader (or a middle aged reader with decrepit eye sight) needs to zoom in on the text. As you will see in the screen capture below, the image will pixelate while the text remains crisp.
Zoomed Text vs Zoomed Image
When you combine Unicode with creative CSS, you can see the possibilities for replacing images, including buttons with text. Not only is this more accessible, but it also results in smaller file sizes and is easier to edit.
Hearing Impaired Users
Unicode is actually important for these users because they need to read text captions or transcripts for video and audio. Once you get beyond basic English (e.g. Spanish subtitles)...well you know Unicode will be important.
Motion Impaired Users
For these users, the issue probably isn't so much reading text as being able to input it - which is the job of developers of operating systems and software. For motion impaired users, a good generalization is that keyboard access is better than using the mouse which requires a little more hand control. In the past I've commented on usability of various inputting devices, but since most do rely on key strokes, there are really no major complaints here.
One audience I didn't touch was color deficient vision, but except possibly for the Aztec script (which isn't even in Unicode yet)...it's not too much of an issue.