[css-box-3] Fix links, link to issue.

Author: fantasai

css-box-3/Overview.bs

@@ -48,7 +48,7 @@ Introduction {#intro}
 
 	Note: This module <a href="https://www.w3.org/TR/2018/WD-css3-box-20180731/">originally contained</a>
 	the CSS Level 3 specification prose relating to
-	box generation (now defined in [[css-display-3]),
+	box generation (now defined in [[css-display-3]]),
 	the box model (defined here),
 	as well as block layout (now only defined in [[CSS2]] Chapters 9 and 10).
 	Since its maintenance was put aside during the development of CSS2.1,
@@ -367,6 +367,10 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 	when they adjoin a <a>fragmentation break</a>
 	(page break / column break / etc.).
 
+	ISSUE: Define how this property affects margins at breaks
+	if the box establishes a <a>fragmentation context</a>.
+	See also <a href="https://github.com/w3c/csswg-drafts/issues/3314">Issue 3314</a>.
+
 Padding {#paddings}
 =======