As already mentioned by Pablo WYSIWYG Web Builder is not a HTML Editor so it not possible to manually edit the HTML from within the software. The only way to edit the HTML is to open the source code externally with the likes of Notepad make the changes to it save and publish the page. Note you will need to repeat this if again if you reopen the project ,(.wbs) with the software again and save this is due to the HTML being overwritten. Also note that this cannot be made possible in any future updates to make it possible to edit the HTML as it only exists once you publish.
What you may be able to do is make a suggestion for an option for the circumstances where you may need it. So, the resultant html at publish time will be what you want
But you have to be very prescriptive and provide a good case and examples of where it will be needed.
However the override html code generated by web builder is helpful in other scenarios. A list that would replace generated code to user code for a given page would be very nice.