function makeSetMarkup(markup) {
	  // NOTE: Null values reduce hidden classes.
	  return {
	    type: 'SET_MARKUP',
	    content: markup,
	    fromIndex: null,
	    fromNode: null,
	    toIndex: null,
	    afterNode: null
	  };
	}
Make an update for setting the markup of a node.

@param {string} markup Markup that renders into an element.
@private