Add IDL for all attributes (#615)

Co-authored-by: Mason Freed <masonf@chromium.org>

research/src/pages/popup/popup.research.explainer.mdx

@@ -166,6 +166,15 @@ When the `popuptoggletarget`, `popupshowtarget`, or `popuphidetarget` attributes
 
 These attributes are only supported on buttons (including `<button>`, `<input type=button>`, etc.) as long as the button would not otherwise submit a form. For example, this is not supported: `<form><input type=submit popuptoggletarget=foo></form>`. In that case, the form would be submitted, and the pop-up would **not** be toggled.
 
+The declarative trigger attributes can also be accessed via IDL:
+
+```javascript
+// These set the IDREF for the target element, and not the element itself:
+myButton.popUpToggleTarget = idref;
+myButton.popUpShowTarget = idref;
+myButton.popUpHideTarget = idref;
+```
+
 ### Javascript Trigger
 
 To show and hide the pop-up via Javascript, there are two methods on HTMLElement:
@@ -209,6 +218,12 @@ Note also that more than one `manual` pop-up can use `defaultopen` and all such
 <div popup=manual defaultopen>Also shown on page load</div>
 ```
 
+The `defaultopen` content attribute can also be accessed via IDL:
+
+```javascript
+myDiv.defaultOpen = true;
+```
+
 ### CSS Pseudo Class
 
 When a pop-up is open, it will match the `:open` pseudo class:
@@ -427,6 +442,13 @@ A new attribute, `anchor`, can be used on a pop-up element to refer to the pop-u
 2. The referenced anchor element could be used by the **Anchor Positioning feature** ([(dated) explainer](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/CSSAnchoredPositioning/explainer.md); [more up-to-date draft spec](https://tabatkins.github.io/specs/css-anchor-position/)).
 
 
+The anchor attribute can also be accessed via IDL:
+
+```javascript
+// This sets the IDREF for the anchor element, and not the element itself:
+myPopUp.anchor = idref;
+```
+
 ## Backdrop
 
 Akin to modal `<dialog>` and fullscreen elements, pop-ups allow access to a `::backdrop` pseudo element, which is a full-screen element placed directly behind the pop-up in the top layer. This allows the developer to do things like blur out the background when a pop-up is showing: