The division operator `/` from Sass is deprecated because `/` is used in
CSS for uses other than dividing numbers. That's why we were getting
many warnings like:
```
Deprecation Warning: Using / for division outside of calc() is
deprecated and will be removed in Dart Sass 2.0.0.
Recommendation: math.div($line-height, 2) or calc($line-height / 2)
More info and automated migrator: https://sass-lang.com/d/slash-div
margin-top: $line-height / 2;
```
Since using math.div makes the code harder to read and `calc` is
universally supported by all browsers (although the implementation in
Internet Explorer doesn't work in certain cases), we're using `calc`
when assigning the value to a CSS property.
However, we're also using divisions when assigning Sass variables, and
in those cases using `calc` is trickier because sometimes these
variables are used in other operations. We'll handle these cases in the
next commit.
We were using a focus outline on links, but weren't doing the same for
buttons. Since sometimes browsers use a default outline which is barely
visible, this was very disorienting when browsing using the keyboard; we
were navigating through links that clearly indicated where the keyboard
focus was, and when reaching a button suddenly we had this almost
imperceptible feedback. Even if I'm used to it, my first reaction is
always "where did the focus go?" until I realize it's now on a button.
This is even more confusing because we've got buttons looking like links
and links looking like buttons.
Note that in the rules for the `:focus` styles we're including buttons
and the `[type="button"]` attribute. This seems redundant since those
styles are already covered by the `button` selector. However, Foundation
adds styles to buttons with the `[type]` attribute. Since the attribute
selector has precedence over the tag selector, we need to use the
attribute selector as well in order to override Foundation's styles.
