Similarly to how text editors often offer variable name auto-completion, it would be useful if the rule-view auto-completed CSS variable names. Say you start typing "var(--" as a value of a property in the rule-view, it'd be nice if the rule-view then showed the usual auto-completion drop-down list (which we already have for property names and values in certain cases), populated with matching variable names if they are in scope. Getting the list of variable names to populate the drop-down shouldn't be too hard since we only need to start looking at the current rule and then walk up the parent rules. This is exactly what the rule-view already does when it displays the list of rules for a given element. The hard(er) part will be making the suggestion drop-down appear at the right time and place.

VS Code added this recently: https://twitter.com/dstorey/status/740042214366027781

Chrome added this recently: https://twitter.com/malyw/status/826486604609028098