Improve font stack with standard font families

[robbertkl]

Hi! This change extends the current font stack for both sans-serif and serif with new "standard font families". Please see: https://www.w3.org/TR/css-fonts-4/#ui-serif-def

For the default font and font-sans class/variants this won't have much effect for now, since system-ui and -apple-system are already included. For font-serifand font-mono it now includes ui-serif / ui-monospace, for which support from Safari landed just last month with Safari 13.1.

To summarize the change:

• The sans-serif font stack is prepended with ui-sans-serif, which makes it:

ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"

• The serif font stack is prepended with ui-serif, which makes it:

ui-serif, Georgia, Cambria, "Times New Roman", Times, serif

• The monospace font stack is prepended with ui-monospace, which makes it:

ui-monospace, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace

Thanks and keep up the good work! Tailwind CSS is awesome 🚀

[jhauraw]

Maybe ui-mono too?

[robbertkl]

Good catch, that should definitely be included as well 😄.

Will add it later today.

[robbertkl]

Maybe ui-mono too?

It's actually ui-monospace.

So I've now added ui-monospace to the mono font stack and also decided to drop -apple-system-ui-serif (and not add -apple-system-ui-monospace) since they were never a main part of Safari. So now this PR is just about adding ui-sans-serif, ui-serif and ui-monospace to the respective font-family classes. I'll edit the PR description accordingly.

[adamwathan]

Merged manually to resolve conflicts, not sure why GitHub doesn't pick it up :/ 2c6f237

Thanks!