The polyfill.js is a popular open source library to support older browsers. 100K+ sites embed it using the cdn.polyfill.io domain. Notable users are JSTOR, Intuit and World Economic Forum. However, in February this year, a Chinese company bought the domain and the Github account. Since then, this domain was caught injecting malware on mobile devices via any site that embeds cdn.polyfill.io. Any complaints were quickly removed (archive here) from the Github repository.
You really can’t. If it was only HTML and CSS, any accessibility program would be able to select any part of the page, and easily alter the CSS and HTML. That is next to impossible now because of JavaScript.
It shouldn’t be up to the website developer. It should be up to the browser developer. You don’t blame a lemmy instance for poor accessibility with Jerboa.