score:2

Accepted answer
public static string tohtml(list<int> tags)
{
    var found = values.where(v => tags.contains(v.key))
                      .select(v => v.value.tohtml());

    return found.any()
        ? string.join("", found.toarray()) + " "
        : string.empty;
}

score:0

const char space = ' ';
var s = values
            .select(v => tags.contains(v.key) ? v.value : string.empty)
            .aggregate((v1, v2) => v1 + space + v2)
            .trimend(space);

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