test

    for (const auto &kv : cssProps) {

        if (kv.first.compare(0, 2, "--") == 0) env[kv.first] = kv.second;

    }

    for (auto &kv : cssProps) {

        kv.second = libhtmlpp::resolveCSSVariables(kv.second, env);

    }

    for (auto it = cssProps.begin(); it != cssProps.end(); ) {

        if (it->first.compare(0, 2, "--") == 0) it = cssProps.erase(it);

        bool unresolved = false;

        it->second = libhtmlpp::resolveCSSVariables(it->second, env, &unresolved);

        // An unresolvable var() (cyclic, or absent with no fallback)

        // invalidates the whole declaration per CSS semantics -- keeping it

        // would emit syntactically-broken text (e.g. a `calc(var(--fs-min)

        // * 1px)` whose `--fs-min` never made it into `env` collapsing to

        // `calc( * 1px)`) instead of just dropping the property so it falls

        // back to inherited/initial, same as a real browser would.

        if (unresolved || it->first.compare(0, 2, "--") == 0) it = cssProps.erase(it);

        else ++it;

    }

    return env;