Can't see borders on text boxes in safari - so don't know where to click

I’m looking for a little help, please!

When I’m using some sites on safari on my mac, I can’t see any borders around (some) text boxes so I don’t know where they are, or where to click. In the picture there’s a text box in the middle - between the “GetResponse API Key” text and the “You can find …” text - you can see the border. But there’s no visible border when I’m just looking at it on safari and I only know the text box is there because I clicked there, hoping to find a text box.

(Weird that it’s in the screenshot but not on the screen)

As you can imagine, this is reallllllllly confusing.