Because of https://www.w3.org/Bugs/Public/show_bug.cgi?id=21242, we can write nested @host @-rules. However it would be better to clarify how nested @host @-rules work.

For example, suppose that we have the following tree:

<hostA>
   #shadow-root
      <hostB>
         #shadow-root
            <style>
            @host { @host { * { border: 1px solid green; } } }
            </style>
            ....

So how should the above nested @host @-rules work?

I think, at least, we have four options:

(a) The above nested @host @-rules should be applied to hostA,

(b) The above nested @host @-rules should be disallowed. So treat as "@host { * { border: 1px solid green; }". hostB should have 1px solid green border.

(c) The above nested @host @-rules should be disallowed. Ignore.

(d) Other options...

I would like to know which would be the best option.