
In «a="b=c"; $a», the '=' sign in the expansion of $a is not active. Therefore, prevent the expansion of $a from being considered an assignment. Update test expectations accordingly.
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES
keys) each highlighter defines.
Refer to the documentation on highlighters for further information.