Despite my doubts, I support this idea.
You have chosen a very good script (light, no dependencies). If you provide the configuration (disabling / enabling options, changing the icon) it will be a useful plugin.
Also consider a version for Joomla 3.x.
Now it works well.

Thanks for this plugin.
I have tested this plugin. I noticed the following:

1. If I see that correctly, then the a11y toolbar is currently only displayed in the back end. Should not it also be displayed in the front end?
2. If I make a change in the a11y toolbar for a site and then navigate to another site using a link, then my a11y settings are lost again. I have to set everything again. That's annoying.
3. The possibility to change the font size is unnecessary - in my opinion. This can be done using the browser.
4. Invert colors and text to speech worked great. For text to speech I used the plugin Read aloud (https://addons.mozilla.org/de/firefox/user/13863253/).
5. Underline links do not underline text within a button.

1. It can be - I would do it as an option in the plugin like the skipto plugin
2. No idea. That would have to be reported to the js script upstream
3. Everything can and should be done in the browser which is why I am not a fan of these scripts but if we have the plugin I see no reason not to and as @zwiastunsw requested everything will be configurable by the user
4. good
5. That is expected. They are not links so they should not be underlined.

Does that plugin bring features that people that need a11y don't have in their browsers / screenreaders / OS already? I think zooming and text to speech is probably not needed because it's included in the browser / Os / Screenreader. As we want to follow AA a11y standards, all links should be underlined anyway so inverting colors and gray hue are probably the only really needed options?
If so, can it be included as a link before everything else? I could think of, that it should be the very first and not the last element on the website.

try it yourself and see

zooming and font size are different things

(I am not a fan of these tools but other people are - so I am offering it for evaluation.)

Let us be clear:

1. Such a plugin is useful and may improve the accessibility of the site, provided that it works well. Brian has chosen a very good script that doesn't cause the errors that widgets of this type cause.
2. The plugin does not solve the problems of people with disabilities. People with disabilities use specialised tools (screen readers, screen magnifiers, braille embossers… etc.). But it improves usability for the elderly, people with visual impairments, people with cognitive problems.
3. E.g. a blind person will use a screen reader and not the option to change text to speech. But this is a very good solution for the elderly, for people who have difficulty concentrating, and also for those who cannot read at the moment.
4. Text enlargement is very useful for many seniors and people who see less well. Such people often do not know about the possibilities of the browser and like to use such tools.
5. The contrast version will not replace the correct contrasts on the website. But it can be helpful for people who have weaker eyesight or for people who browse the site, for example in a sunny place.
6. WCAG does not require the links to be underlined (they must be highlighted). But underlined links can improve accessibility for visually impaired people or for people who have difficulty concentrating.

If the JAT Team decides to use that, I am fine with it - Just want to avoid to "clutter" the backend with tools, that browsers already provide.

It's a plugin that can be disabled
Your design has a link called accessibility options

@wilsonge do I continue with this and make it configurable, translatable and RTL aware or not?

Go for it. A11y's happy so I'm happy. Script seems fine

@wilsonge looks like this script is a no-go. Any settings you make do not persist when you change page

That's a shame :( given the repo still hasn't even merged the simple typo PRs from late last year I guess there's no point in raising any of this with him? We could fork, find a new one or decide surplus to requirements.

I have been looking for alternatives but no joy yet. They are either paid, include remote files or are jquery. I will keep looking.

I have submitted an issue to the repo. They did do a website update a few days ago so maybe it will get a response

Looking into this a bit more and we can actually do most of it with css (everything but the audio) and would only need a script to apply the css and make it persistent across the session

I am still wanting to do this but am putting it on the "back-burner" for now

@wilsonge clearly this javascript is a no go as they have not replied to any questions. :(

I see two options moving forward

1. try to write a similar javascript
2. add a new user plugin with the same options

The advantage of 2 is that it will be much easier to maintain and create AND instead of it being a setting per browser/computer it is a setting per user as the settings will be stored in the database.

@wilsonge Thoughts?

closed see #26194