I think I’m going to bow out because it’s clear this discussion isn’t going anywhere, but you should look up what transclusion means because it makes no sense to say that you can do it in CSS.
To do the same thing in CSS you would do <div class="myelement"><div class="slot"><div class="a">Transcluded content</div></div></div> @scope (.myelement) to (.slot) { /* rules that apply to .myelement but not .a */ }).