Add link to CSSWG 6965 (#618)

It was [mentioned](whatwg/html#7785 (comment)) that this CSSWG issue was helpful, so this adds a direct link.

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

research/src/pages/popup/popup.proposal.alternatives.mdx

@@ -5,7 +5,7 @@ showInMenu: false
 ---
 
 - [@mfreed7](https://github.com/mfreed7)
-- March 16, 2022
+- October 11, 2022
 
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@@ -271,6 +271,7 @@ This approach, on its face, seems very simple and elegant. However, there are tw
 
 For these two reasons, this approach was abandoned in favor of the [current HTML attribute proposal](#html-content-attribute).
 
+Note that the CSS Property alternative was also discussed at some length by the CSSWG in [Issue 6965](https://github.com/w3c/csswg-drafts/issues/6965).
 
 ### API Shape