[mediaqueries] Add date to historical retrospective.

Closes w3c#5339

Author: frivoal

mediaqueries-4/Overview.bs

@@ -71,7 +71,7 @@ Introduction</h2>
 
 	<em>This section is not normative.</em>
 
-	HTML4 [[HTML401]] defined a mechanism to support media-dependent style sheets,
+	In 1997, HTML4 [[HTML401]] defined a mechanism to support media-dependent style sheets,
 	tailored for different <a>media types</a>.
 	For example, a document may use different style sheets for screen and for print.
 	In HTML, this can be written as:

mediaqueries-5/Overview.bs

@@ -65,7 +65,7 @@ Introduction</h2>
 
 	<em>This section is not normative.</em>
 
-	HTML4 [[HTML401]] defined a mechanism to support media-dependent style sheets,
+	In 1997, HTML4 [[HTML401]] defined a mechanism to support media-dependent style sheets,
 	tailored for different <a>media types</a>.
 	For example, a document may use different style sheets for screen and for print.
 	In HTML, this can be written as: