Composer's source view is helpful in debugging HTML. Also
helpful is the DOM view in the status bar at the bottom of the window:
watch it change as you click on parts of the document. Clicking
on a particular item in the status bar DOM view will select it.
DOM Inspector: a separate window, which shows the DOM (Document
Object Model) as a tree and allows selecting items and getting
information about them.
can be a huge help in debugging JS pages.