Fix antialiasing under Safari

Bigfoot uses CSS animations to show its popovers, and in OS X Safari this prevents the footnote text from being rendered with subpixel antialiasing. Since subpixel antialiasing is the default antialiasing style, footnote text ends up looking thinner than body text. This change fixes that.

(If you have set `-webkit-font-smoothing: antialiased` on your body text, you can use the rule

    .footnote-content-wrapper {
        -webkit-font-smoothing: antialiased;
    }

to make the footnotes look like the body text again.)