Add mw-parser-output class to Parsoid's output

* Templates will target mw-parser-output
* Skins will target mw-body-content
* There is a large overlap between the targets of the
  two classes, but mw-body-content is also applied to
  interface elements not present in the parser output.

Bug: T37247
Bug: T164792
Change-Id: I05476df24f06a6b06f37a6a22d3e33ef4a939ef8

lib/wt2html/DOMPostProcessor.js

@@ -374,12 +374,16 @@ DOMPostProcessor.addMetaData = function(env, document) {
 	document.body.setAttribute('dir', dir);
 
 	// Set 'mw-body-content' directly on the body.
-	// This the designated successor for #bodyContent in core skins.
+	// This is the designated successor for #bodyContent in core skins.
 	document.body.classList.add('mw-body-content');
 	// Set 'parsoid-body' to add the desired layout styling from Vector.
 	document.body.classList.add('parsoid-body');
 	// Also, add the 'mediawiki' class.
+	// Some Mediawiki:Common.css seem to target this selector.
 	document.body.classList.add('mediawiki');
+	// Set 'mw-parser-output' directly on the body.
+	// Templates target this class as part of the TemplateStyles RFC
+	document.body.classList.add('mw-parser-output');
 };
 
 DOMPostProcessor.prototype.doPostProcess = function(document) {