system-font, system-ui, -apple-system and ... is «missing fonts»
Hello.

How about to support font names by CSS4?

It needs because now so many sites uses «System Font» in css `font-family`.
After that `npm run inject <sitename>` returns TRASH in width/height values of text blocks.

Okay, even if you try use "replace font", you don't know where is Regular, Bold or Italic style.. Anyway sad...

by this https://github.com/brainly/html-sketchapp-example.

Maybe is real to replace "system-ui" and other parallels with `page2layers`?

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Screens by sketchapp.com

![image](https://user-images.githubusercontent.com/25253501/36632108-a573ed28-19b4-11e8-838e-366e2cc78e0a.png)

![image](https://user-images.githubusercontent.com/25253501/36632114-c17ad69e-19b4-11e8-86ec-f711b0b57bdc.png)

![image](https://user-images.githubusercontent.com/25253501/36632115-c5d5454e-19b4-11e8-83e7-833e612eb729.png)

![image](https://user-images.githubusercontent.com/25253501/36632131-e6bcf6da-19b4-11e8-8ffb-a52ba6e40bb9.png)
