If the text is in a Joomla article, they can use the edit button on the front end, change the text and hit save. That is even easier that editing a separate file. BTW, most of the text editor in Joomla are WYSIWYG that don't require HTML knowledge.
Mark Giambruno wrote:
I'm looking for a solution that uses only HTML or an extension,...
I thought you said you wanted something that doesn't require HTML knowledge