`<em>` in interactive examples doesn't render as italic in Safari

[caugner]

Reported via mdn/content#42992 (comment).

STR:

1. In Safari, open https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/em#try_it

Expected:

Actual:

Note: This is similar to mdn/yari#7203, but content is not affected, because Fred uses multiple Inter @font-faces, whereas the Playground Runner uses a single @font-face.

PS: It works in the live samples further down, because those don't use Inter.