Trying to run the code examples from Book and Rust-by-example frequently result in lots of errors

[eduarddejong]

See the screenshot below (sorry for some Dutch language, I will clarify things if desirable).
This is my experience using Firefox (on Linux).
I now decided to open de dev tools console to see what errors it's reporting, and it's not good.

I know these books are clearly using the Playground via some API, that's why I am reporting it here and not for each of those books.

Also note that the first time I noticed those timeout problems was close to the beginning this year, when I was reading the Book for the first time. So my impression is that the problems are not new at all.

But what I now noticed is that in many cases the Ctrl+Enter also fails to execute completely (nothing happens), while pressing the run button with the mouse still works.

[shepmaster]

Duplicate of #955