I built text sizing widgets for years, every damn site. I was so proud of them too. It was a way of showing I care about users. It was all ego. As soon as I could start tracking clicks on those widgets I found they were not used. Even on sites I built for low vision communities.
Instead, a good design with non-hardcoded typefaces that is responsive and does not disable zoom was all I needed. In short, good development techniques and best practices. That handled most of my edge cases just fine. For the remainder, a little documentation in the form of simple, contextual help text.
Notice I am not referencing assistive technology. For the most part, those users don’t need your widgets, they have already obtained tools to work around a non-inclusive web.
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Instead of custom widget, maybe help educate users on how to use their own web browser. Perhaps link to, or offer as pop-up help, quick instructions on how to scale text in the user’s current browser.
Now you will have educated a user. You will have armed a user to have a better experience across the whole of the web. You will have stopped selfishly building a widget for just your users and contributed to empowering all users.
What could be more inclusive than that?