> They have long had measures in place to detect invisible text (eg. white text on white background) or hidden text (where HTML elements are styled over other HTML elements). This suggests both CSS rendering and JS rendering.

No, this actually suggests it's not doing either. Both invisible and hidden text the way you've described it would be implemented with a CSS style. Not using that style would mean the text would appear as normal. I understand you probably meant that the JS was injecting the text in, which is fully possible, but that's neither hidden nor invisible text.