You're correct that "standout" doesn't match the terminology in
ECMA-48. The use of "standout" was chosen to follow the terminology
in prompt strings, I believe.
This matched the behavior of most terminals at the time (notice
${(k)color[(r)standout]} is actually from Sven all the way back in
2001). Also, there's no way to directly reference an escape sequence
for any of the "Attribute Codes" (they aren't in the $fg / $bg
hashes), so those values are mostly there for commentary value.
I would have no objection to adding "italic"/"no-italic" to the color
hash. It also appears standout would more accurately be tied to
reverse-video now, but because the keys of the hash are the numeric
codes we can only have one or the other.
Would it be possible to drop `standout` (as it's rather ambiguous) and instead add both `italic` and `reverse`? They both have `terminfo` entries.