I think it's safe to assume most big players have browser rendering enabled (I hope so). imo AI is struggling with a lot of languages that are not as popular as javascript, mostly because it's more niche and you don't get a lot of good examples on the web.
In my experience, coding agents seem to do a normal fetch when provided links. Which makes sense — headless browser automation is expensive, and only really necessary for interacting with a webpage.
But with these RLHF'd AIs, being confident and helpful as they are, it took me a while to realize that they couldn't actually read the Apple developer links I was giving them. Like a kid who can't read the chalkboard, but doesn't realize they need glasses.
It's expensive for sure, but probably a drop in the water compared to the cost of renting H100. Plus, it would be a massive boost in terms of data quality/quantity for them. But maybe you are right, I'm just surprised it's not the case