Why are websites trying to talk at me?

Why am I seeing this on so many sites recently (in FireFox)? I've seen it half a dozen times in the past month or so, had never noticed it before then.

"You can't use speech synthesis because the speech dispatcher library is missing"

There's a 'learn more' link, but it just talks about getting speech synthesis working in my browser. Searching for the error string returns similar discussions. I don't want to do that (especially if it's some new marketing fad). Obviously it's speech synthesis but *what is it saying?*

The most recent instance was a dell.com product page posted here on HN:

https://www.dell.com/en-us/shop/dell-ultrasharp-52-thunderbolt-hub-monitor-u5226kw/apd/210-bthw/monitors-monitor-accessories

As lots of people on HN will have visited that link I thought somebody might be able to tell me what they heard?