Why is it that ::first-letter only applies to blocks?

@sgalineau @robinberjon Post to www-style. If @davidbaron and @bz_moz say it's okay to change, then we can probably update it for L4.

@fantasai @sgalineau @robinberjon @davidbaron Other than performance concerns, I see nothing obviously impossible about this.

@bz_moz @fantasai @sgalineau @robinberjon But ::first-letter seems like one of those things that takes a lot more code than it's worth.