Landing pull request 509. Add a comment explaining the use of...

Landing pull request 509. Add a comment explaining the use of document.getElementsByTagName('body')[0] instead of document.body.

More Details:
 - https://github.com/jquery/jquery/pull/509

src/support.js

@@ -122,7 +122,7 @@ jQuery.support = (function() {
 		div.cloneNode( true ).fireEvent( "onclick" );
 	}
 
-	// Check if a radio maintains it's value
+	// Check if a radio maintains its value
 	// after being appended to the DOM
 	input = document.createElement("input");
 	input.value = "t";
@@ -142,7 +142,11 @@ jQuery.support = (function() {
 	// Figure out if the W3C box model works as expected
 	div.style.width = div.style.paddingLeft = "1px";
 
-	body = document.getElementsByTagName( "body" )[ 0 ];
+	// We don't want to do body-related feature tests on frameset
+	// documents, which lack a body. So we use
+	// document.getElementsByTagName("body")[0], which is undefined in
+	// frameset documents, while document.body isn’t. (7398)
+	body = document.getElementsByTagName("body")[ 0 ];
 	// We use our own, invisible, body unless the body is already present
 	// in which case we use a div (#9239)
 	testElement = document.createElement( body ? "div" : "body" );
@@ -157,8 +161,8 @@ jQuery.support = (function() {
 	if ( body ) {
 		jQuery.extend( testElementStyle, {
 			position: "absolute",
-			left: "-1000px",
-			top: "-1000px"
+			left: "-999px",
+			top: "-999px"
 		});
 	}
 	for ( i in testElementStyle ) {