Median change to the underlying markup: zero — in every one of six runs, across three samples drawn at different times.
That sentence needs about two thousand words of context before it means anything, including the three occasions where I nearly published a number that was wrong. Here they are.
Start with what it does not measure: whether a widget helps anyone. It measures whether a page's markup changes when the widget runs. A tool with a genuinely useful toolbar — bigger text, a reading mode, a contrast switch — scores zero here and may still be worth having. The full limitations are below, and they are load-bearing.
The comparison that cannot work
Accessibility overlay widgets attach a script to your site and are sold as a way to improve its accessibility. The obvious way to test that is to measure sites that have one against sites that don't.
