css button font size doesn't work

Indeed this only happens on WebKit-MacOS based browsers. Seems to be a WebKit restriction so that the Aqua appearance stays always so.

As long as the Aqua appearance is enabled for push buttons, certain CSS properties will be ignored. Because Aqua buttons do not scale, the height property will not be honored. Similarly font and color customizations will also not be honored. The overriding principle for push buttons is that you will never see a button that is some “half-Aqua” mix. Either the button will look perfectly native, or it will not be Aqua at all.

Source: https://www.webkit.org/blog/28/buttons

Which explains why setting a background makes font-size works; it breaks the Aqua appearance.

@pzin's response got me started on the right track. He's right in that anything that breaks aqua will get it done. The recommended way to handle it without having to specify a background color is this bad boy:

-webkit-appearance: button;