This stylelint-scss rule is useful because we were inconsistent when
using calc(); sometimes we added interpolation to Sass variables, and
sometimes we didn't. The reason why we originally added interpolation
was that it was necessary until we migrated to Dart Sass in commit
d54971e53. Since then, we can omit the interpolation, which is also what
the Sass documentation recommends [1].
[1] https://sass-lang.com/documentation/values/calculations/
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: $line-height / 2;
```
Since using math.div makes the code harder to read and `calc` is
universally supported by all browsers (although the implementation in
Internet Explorer doesn't work in certain cases), we're using `calc`
when assigning the value to a CSS property.
However, we're also using divisions when assigning Sass variables, and
in those cases using `calc` is trickier because sometimes these
variables are used in other operations. We'll handle these cases in the
next commit.
This way we simplify the code a bit.
Note we're only using this function when variables for background colors
are already defined, since that means customizing the variable using the
background color will automatically change the color of the text.
Customization isn't easier when using raw colors.
We were defining (for instance) white text against the `$brand`
background. That meant that, if somebody customized the `$brand` color
so it used a light color, they had to customize the text color as well
in order to guarantee proper contrast between text and background
colors.
So we're using `color-pick-contrast` instead, which means we don't have
to manually calculate whether white or black will be the color which
makes the text more readable.
Note we require extra <span> tags because the <progress> tag is an empty
tag (like <img>), and so it can't have ::before or ::after
pseudo-elements. There's a workaround for that, but currently it only
works on Chrome.
For some reason, the text seems to be slightly misaligned vertically in
all implementations I've tried. So the `top: -0.1rem` rule is a hack to
align it properly.
The selector `nth-child(even)` didn't need specific rules, and it's
easier to understand the code for the selector `nth-child(odd)` if all
breakpoints are grouped together.
