Ampersand rendered incorrectly in rust playground.

[moxian]

(originally filed as rust-lang/rust#42231)

In https://play.rust-lang.org/ ampersands are rendered as thin tall black rectangles - see http://i.imgur.com/7EDtQD9.png

This is the same as #47, but given the spread of the issue, I don't think its feasible asking everyone to fix the issue locally - that just does not scale.

This also sounds exactly like rust-lang/rust#24355, but this time in playground rather than in rust docs. Would it be feasible to apply the same fix here (forbid usage of local font)?

(Win7, Firefox 53.0.3 64 bit)