Shadow Roots and Inheritance

There is a helluva gotcha with styling a <details> element, as documented here by Kitty Guiraudel. It’s obscure enough that you might never run into it, but if you do, I could see it being very confusing (it would confuse me, at least).

Perhaps you’re aware of the shadow DOM? It’s talked about a lot in terms of web components and comes up when thinking in terms of <svg> and <use>. But <details> has a shadow DOM too:

<details>
  #shadow-root (user-agent)
  <slot name="user-agent-custom-assign-slot" id="details-summary">
    <!-- <summary> reveal -->
  </slot>
  <slot name="user-agent-default-slot" id="details-content">
    <!-- <p> reveal -->
  </slot>

  <summary>System Requirements</summary>
  <p>
    Requires a computer running an operating system. The computer must have some
    memory and ideally some kind of long-term storage. An input device as well
    as some form of output device is recommended.
  </p>
</details>

As Amelia explains, the <summary> is inserted in the first shadow root slot, while the rest of the content (called “light DOM”, or the <p> tag in our case) is inserted in the second slot.

The thing is, none of these slots or the shadow root are matched by the universal selector *, which only matches elements from the light DOM. 

So the <slot> is kind of “in the way” there. That <p> is actually a child of the <slot>, in the end. It’s extra weird, because a selector like details > p will still select it just fine. Presumably, that selector gets resolved in the light DOM and then continues to work after it gets slotted in.

But if you tell a property to inherit, things break down. If you did something like…

<div>
  <p></p>
</div>

div {
  border-radius: 8px;
}
div p {
  border-radius: inherit;
}

…that <p> is going to have an 8px border radius.

But if you do…

<details>
  <summary>Summary</summary>
  <p>Lorem ipsum...</p>
</details>

details {
  border-radius: 8px;
}
details p {
  border-radius: inherit;
}

That <p> is going to be square as a square doorknob. I guess that’s either because you can’t force inheritance through the shadow DOM, or the inherit only happens from the parent which is a <slot>? Whatever the case, it doesn’t work.

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source https://kittygiraudel.com/2021/08/23/shadow-roots-and-inheritance/