"3.6 Satisfying Matching Criteria"

Consider following case. Custom Element that extends ordered list(, and creates a pyramid of news)
my-article-list shadow root:

<div class="lead">
    <h1>Breaking news:</h1>
    <content select=":first-child"></content>
</div>
<div class="asides">
    <content select=":nth-child(2)"></content>
    <content select=":nth-child(3)"></content>
</div>
<div>
    <content></content>
</div>

And input should look like:

<my-article-list>
    <article>Most important article</article>
    <article>Important article</article>
    <article>Important article</article>
    <article>article</article>
    ...
</my-article-list>


Without those pseudo-classes my-articles-list needs to modify light DOM, or at least CSS properties of child nodes, which seems very intrusive to me.

Use of ".first", ".second", ... classes also does not seem right. As the user of my-article-list should now be aware how long should he count, and as for ordered list nodes order already serves this information.

I know that it was removed via 21924, but unfortunately I have no idea how hard it would be to implement it, and how it could affect performance. I just guess it could be not much worse than `:not()`.