Transclusion can totally be done with CSS now. That’s a donut selector.
The other things are just plain old JavaScript and have nothing to do with Shadow DOM. Yes, they can be done with customElement, but they can also just be done without it.
The other things are just plain old JavaScript and have nothing to do with Shadow DOM. Yes, they can be done with customElement, but they can also just be done without it.
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 */ }).