[Web Accessibility Properties] outline how this can be used to explain existing elements

[domenic]

I think this is implicit in your design, but it would be good to make it explicit. For example it would be great to show how to implement e.g. <img>. My understanding is it would be something like this:

// In UA code setting up a new <img>
this.addEventListener("ariarequest", ev => {
  if (ev.attributeName === "labelledby") {
    if (this.hasAttribute("alt")) {
      ev.attributeValue = this.getAttribute("alt");
    }
    if (this.hasAttribute("title")) {
      ev.attributeValue = this.getAttribute("title");
    }
  } else if (ev.attributeName === "describedby") {
    if (this.hasAttribute("title")) {
      ev.attributeValue = this.getAttribute("title");
    }
  } else if (ev.attributeName === "role") {
    if (this.hasAttribute("alt") && this.getAttribute("alt").trim() !== "") {
      ev.attributeValue = "img";
    } else {
      ev.attributeValue = "presentation";
    }
  }
});