From 9c77efd94a254a354a551274251b6f351373f9d4 Mon Sep 17 00:00:00 2001
From: Simon Pieters <zcorpan@gmail.com>
Date: Wed, 27 Oct 2021 00:02:18 +0200
Subject: [PATCH] Allow setting rootMargin

Part of #428.
---
 index.bs | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/index.bs b/index.bs
index 9018d65..e31f37d 100644
--- a/index.bs
+++ b/index.bs
@@ -218,7 +218,7 @@ use-cases. Per-{{observe()}} options could be provided in the future if the need
 interface IntersectionObserver {
 	constructor(IntersectionObserverCallback callback, optional IntersectionObserverInit options = {});
 	readonly attribute (Element or Document)? root;
-	readonly attribute DOMString rootMargin;
+	         attribute DOMString rootMargin;
 	readonly attribute FrozenArray&lt;double&gt; thresholds;
 	undefined observe(Element target);
 	undefined unobserve(Element target);
@@ -279,7 +279,14 @@ interface IntersectionObserver {
 		this is not guaranteed to be identical to the |options|.{{IntersectionObserverInit/rootMargin}}
 		passed to the {{IntersectionObserver}} constructor.  If no
 		{{IntersectionObserverInit/rootMargin}} was passed to the {{IntersectionObserver}}
-		constructor, the value of this attribute is "0px 0px 0px 0px".
+		constructor, and the {{IntersectionObserver/rootMargin}} setter has not been invoked,
+		the value of this attribute is "0px 0px 0px 0px".
+
+		On setting, attempt to <a>parse a root margin</a>
+		from the given value.
+		If a list is returned,
+		set |this|'s internal {{[[rootMargin]]}} slot to that.
+		Otherwise, <a>throw</a> a {{SyntaxError}} exception.
 	: <dfn>thresholds</dfn>
 	::
 		A list of thresholds, sorted in increasing numeric order,