The Custom HTML field allows for simpler HTML code with a smart WYSIWYG editor. You can embed different elements such as:
If you are editing a Custom HTML field in </> Source-mode and hit the Save button without leaving </> Source-mode (by clicking the button again), these changes will not be saved!
Try to always remember to click the </> Source button when leaving this mode, before saving the field. Otherwise the risk is that the changes will be lost.
If you want to see more of the source-code when editing you can drag the corner of the box to increase the height of it.
There is also an easy way to implement predefined Alerts to your forms available from Bootstrap that you could put in the Custom HTML field.