From e835f04c13a928ca5aa665e43bbb524a50aa590b Mon Sep 17 00:00:00 2001
From: Darren Shen <shend@chromium.org>
Date: Mon, 19 Feb 2018 12:43:36 +1100
Subject: [PATCH 1/2] computedStyleMap() must return same object

---
 css-typed-om/Overview.bs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/css-typed-om/Overview.bs b/css-typed-om/Overview.bs
index 2d828990..76197382 100644
--- a/css-typed-om/Overview.bs
+++ b/css-typed-om/Overview.bs
@@ -552,6 +552,8 @@ partial interface Element {
 <dfn>Computed StylePropertyMap</dfn> objects
 represent the [=computed values=] of an {{Element}},
 and are accessed by calling the {{Element/computedStyleMap()}} method.
+An {{Element}} must return the same {{StylePropertyMapReadOnly}} object
+for every call of the {{Element/computedStyleMap()}} method.
 
 <div algorithm>
     The <dfn method for="Element">computedStyleMap()</dfn> method must,

From e72f693b68ce127e0589dc8f862fb6e1ba5e0b65 Mon Sep 17 00:00:00 2001
From: Darren Shen <shend@chromium.org>
Date: Wed, 21 Feb 2018 09:16:54 +1100
Subject: [PATCH 2/2] Add explicit caching logic to computedStyleMap()

---
 css-typed-om/Overview.bs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/css-typed-om/Overview.bs b/css-typed-om/Overview.bs
index 76197382..758dfc72 100644
--- a/css-typed-om/Overview.bs
+++ b/css-typed-om/Overview.bs
@@ -552,15 +552,19 @@ partial interface Element {
 <dfn>Computed StylePropertyMap</dfn> objects
 represent the [=computed values=] of an {{Element}},
 and are accessed by calling the {{Element/computedStyleMap()}} method.
-An {{Element}} must return the same {{StylePropertyMapReadOnly}} object
-for every call of the {{Element/computedStyleMap()}} method.
+
+Every {{Element}} has a <dfn attribute for=Element>\[[computedStyleMapCache]]</dfn> internal slot,
+initially set to `null`,
+which caches the result of the {{Element/computedStyleMap()}} method
+when it is first called.
 
 <div algorithm>
     The <dfn method for="Element">computedStyleMap()</dfn> method must,
     when called on an {{Element}} |this|,
     perform the following steps:
 
-    1. Return a live {{StylePropertyMapReadOnly}} object,
+    1. If |this|'s {{Element/[[computedStyleMapCache]]}} internal slot is set to `null`,
+        set its value to a live {{StylePropertyMapReadOnly}} object,
         whose {{[[declarations]]}} internal slot internal slot are the name and [=computed value=] of
         every longhand CSS property supported by the User Agent,
         every registered [=custom property=],
@@ -568,6 +572,8 @@ for every call of the {{Element/computedStyleMap()}} method.
         on |this|,
         ordered by putting CSS-defined properties before custom properties,
         then sorting by code-point order of property name within those two categories.
+
+    2. Return |this|'s {{Element/[[computedStyleMapCache]]}} internal slot.
 </div>
 
 Note: like {{Window/getComputedStyle()|Window.getComputedStyle()}},