CSS uses '-' (minus) as a separator character in property names, but
this is an inconvenient character to use in an R function argument name.
Instead, you can use '.' (period) and/or '_' (underscore) as
separator characters. For example, css(font.size = "12px") yields
"font-size:12px;".
To mark a property as !important, add a '!' character to the end
of the property name. (Since '!' is not normally a character that can be
used in an identifier in R, you'll need to put the name in double quotes or
backticks.)
Argument values will be converted to strings using
paste(collapse = " "). Any property with a value of NULL or
"" (after paste) will be dropped.