In those versions of zsh, «[[ -o nosuchoption ]]» is regarded as a syntax error. In newer zsh versions, it merely returns non-zero (specifically, it returns 3, unlike «[[ -o unsetoption ]]» which returns 1). Fixes #732. Fixes #733.
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES keys) each highlighter defines.
Refer to the documentation on highlighters for further information.