当时在鼓捣 bookdown，尤其是 build others' book。
突然发现 RStudio Viewer panel 无法正常显示 book。
调试一番之后，发现是由于 link to a CSS file out of the output directory.
So I use Rscript
website/bookdown/bookname to add CSS file.
Update: on 11/07, I give up it, maybe I will report the bug someday. Anyway, I decide to stick to
/bookdown.css, because you can see the effect immediately after modifying the CSS file without building again.
Update on 12/07: I saw that RStudio’s View panel only works for local resources. That might explain this issue.