Comments are a great asset to new developers and a great way to place little notes to yourself reminding yourself what pieces of code are doing what. Comments are also great ways to troubleshoot bugs and code errors, as they give you the ability to comment out lines of code one at a time to search for the exact line causing problems.
Comment out elements temporarily rather than removing them, especially if they've been left unfinished.
Write notes or reminders to yourself inside your actual HTML documents.
Create notes for other scripting languages like JavaScript which requires them.
<!-- Opening Comment Tag
-- > Closing Comment Tag