mx_MatrixToolbar_warning should have alt text

[mgifford]

Description

You can either use alternative text here:
<img class="mx_MatrixToolbar_warning" src="img/warning.05cc423.svg" width="24" height="23">

Or add a title in the SVG here:
https://riot.im/app/img/warning.05cc423.svg

Like described here:
https://www.deque.com/blog/creating-accessible-svgs/

Steps to reproduce

I went here:
https://riot.im/app/#/room/!SOMErandomCODE:matrix.org

Ran the WebAim WAVE toolbar & looked at the source code.

[lampholder]

I've put this as p2 because, if I understand correctly, it's the presence of the warning icon that will be undetectable to users using screen readers - they'll still be able to read the text of the warning. Is that right?

[mgifford]

I would add alt="alert" to the <img>. That's probably the easiest. You could argue that it is redundant to the existing text in which case you should use alt="" to tell screen readers that it is a decorative image.

https://webaim.org/techniques/alttext/