The source mode is shown when clicking the HTML icon in the toolbar and lets you see/change the raw HTML source for the content.
Enable source mode
To enable it, make sure "Allow Source Mode" is enabled in the Source section of the configurator, and add
html to your toolbar buttons under Toolbar where you want the button to be available.
If desired, you can enable CodeMirror syntax highlighting for easier use of the source. Under the "Source" section of the configurator, make sure "Enable CodeMirror" is enabled.
Beautifying source code
By enabling Beautify source code in the Source section, the markup can be reformatted for you, adding the proper indentation and newlines and such. This uses js-beautify.
Beautifying the markup will happen at most once every 3 seconds while editing content. It can also be forced by toggling the source view. If you're experiencing performance issues on large pages, please contact support.